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<title>[55203] trunk: Introduce HTML API with HTML Tag Processor</title>
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<dt style="float: left; width: 6em; font-weight: bold">Revision</dt> <dd><a style="font-weight: bold" href="https://core.trac.wordpress.org/changeset/55203">55203</a><script type="application/ld+json">{"@context":"http://schema.org","@type":"EmailMessage","description":"Review this Commit","action":{"@type":"ViewAction","url":"https://core.trac.wordpress.org/changeset/55203","name":"Review Commit"}}</script></dd>
<dt style="float: left; width: 6em; font-weight: bold">Author</dt> <dd>azaozz</dd>
<dt style="float: left; width: 6em; font-weight: bold">Date</dt> <dd>2023-02-03 01:03:59 +0000 (Fri, 03 Feb 2023)</dd>
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<pre style='padding-left: 1em; margin: 2em 0; border-left: 2px solid #ccc; line-height: 1.25; font-size: 105%; font-family: sans-serif'>Introduce HTML API with HTML Tag Processor

This commit pulls in the HTML Tag Processor from the Gutenbeg repository.

The Tag Processor attempts to be an HTML5-spec-compliant parser that provides the ability in PHP to find specific HTML tags and then add, remove, or update attributes on that tag. It provides a safe and reliable way to modify the attribute on HTML tags.

More information: https://github.com/WordPress/wordpress-develop/pull/3920.

Props: antonvlasenko, bernhard-reiter, costdev, dmsnell, felixarntz, gziolo, hellofromtonya, zieladam, flixos90, ntsekouras, peterwilsoncc, swissspidy, andrewserong, onemaggie, get_dave, aristath, scruffian, justlevine, andraganescu, noisysocks, dlh, soean, cbirdsong, revgeorge, azaozz.
Fixes <a href="https://core.trac.wordpress.org/ticket/57575">#57575</a>.</pre>

<h3>Modified Paths</h3>
<ul>
<li><a href="#trunksrcwpsettingsphp">trunk/src/wp-settings.php</a></li>
</ul>

<h3>Added Paths</h3>
<ul>
<li>trunk/src/wp-includes/html-api/</li>
<li><a href="#trunksrcwpincludeshtmlapiclasswphtmlattributetokenphp">trunk/src/wp-includes/html-api/class-wp-html-attribute-token.php</a></li>
<li><a href="#trunksrcwpincludeshtmlapiclasswphtmlspanphp">trunk/src/wp-includes/html-api/class-wp-html-span.php</a></li>
<li><a href="#trunksrcwpincludeshtmlapiclasswphtmltagprocessorphp">trunk/src/wp-includes/html-api/class-wp-html-tag-processor.php</a></li>
<li><a href="#trunksrcwpincludeshtmlapiclasswphtmltextreplacementphp">trunk/src/wp-includes/html-api/class-wp-html-text-replacement.php</a></li>
<li>trunk/tests/phpunit/tests/html/</li>
<li><a href="#trunktestsphpunittestshtmlwpHtmlTagProcessorbookmarkphp">trunk/tests/phpunit/tests/html/wpHtmlTagProcessor-bookmark.php</a></li>
<li><a href="#trunktestsphpunittestshtmlwpHtmlTagProcessorphp">trunk/tests/phpunit/tests/html/wpHtmlTagProcessor.php</a></li>
</ul>

</div>
<div id="patch">
<h3>Diff</h3>
<a id="trunksrcwpincludeshtmlapiclasswphtmlattributetokenphp"></a>
<div class="addfile"><h4 style="background-color: #eee; color: inherit; margin: 1em 0; padding: 1.3em; font-size: 115%">Added: trunk/src/wp-includes/html-api/class-wp-html-attribute-token.php</h4>
<pre class="diff"><span>
<span class="info" style="display: block; padding: 0 10px; color: #888">--- trunk/src/wp-includes/html-api/class-wp-html-attribute-token.php                          (rev 0)
+++ trunk/src/wp-includes/html-api/class-wp-html-attribute-token.php    2023-02-03 01:03:59 UTC (rev 55203)
</span><span class="lines" style="display: block; padding: 0 10px; color: #888">@@ -0,0 +1,89 @@
</span><ins style="background-color: #dfd; text-decoration:none; display:block; padding: 0 10px">+<?php
+/**
+ * HTML Tag Processor: Attribute token structure class.
+ *
+ * @package WordPress
+ * @subpackage HTML-API
+ * @since 6.2.0
+ */
+
+/**
+ * Data structure for the attribute token that allows to drastically improve performance.
+ *
+ * This class is for internal usage of the WP_HTML_Tag_Processor class.
+ *
+ * @access private
+ * @since 6.2.0
+ *
+ * @see WP_HTML_Tag_Processor
+ */
+class WP_HTML_Attribute_Token {
+       /**
+        * Attribute name.
+        *
+        * @since 6.2.0
+        * @var string
+        */
+       public $name;
+
+       /**
+        * Attribute value.
+        *
+        * @since 6.2.0
+        * @var int
+        */
+       public $value_starts_at;
+
+       /**
+        * How many bytes the value occupies in the input HTML.
+        *
+        * @since 6.2.0
+        * @var int
+        */
+       public $value_length;
+
+       /**
+        * The string offset where the attribute name starts.
+        *
+        * @since 6.2.0
+        * @var int
+        */
+       public $start;
+
+       /**
+        * The string offset after the attribute value or its name.
+        *
+        * @since 6.2.0
+        * @var int
+        */
+       public $end;
+
+       /**
+        * Whether the attribute is a boolean attribute with value `true`.
+        *
+        * @since 6.2.0
+        * @var bool
+        */
+       public $is_true;
+
+       /**
+        * Constructor.
+        *
+        * @since 6.2.0
+        *
+        * @param string $name         Attribute name.
+        * @param int    $value_start  Attribute value.
+        * @param int    $value_length Number of bytes attribute value spans.
+        * @param int    $start        The string offset where the attribute name starts.
+        * @param int    $end          The string offset after the attribute value or its name.
+        * @param bool   $is_true      Whether the attribute is a boolean attribute with true value.
+        */
+       public function __construct( $name, $value_start, $value_length, $start, $end, $is_true ) {
+               $this->name            = $name;
+               $this->value_starts_at = $value_start;
+               $this->value_length    = $value_length;
+               $this->start           = $start;
+               $this->end             = $end;
+               $this->is_true         = $is_true;
+       }
+}
</ins></span></pre></div>
<a id="trunksrcwpincludeshtmlapiclasswphtmlspanphp"></a>
<div class="addfile"><h4 style="background-color: #eee; color: inherit; margin: 1em 0; padding: 1.3em; font-size: 115%">Added: trunk/src/wp-includes/html-api/class-wp-html-span.php</h4>
<pre class="diff"><span>
<span class="info" style="display: block; padding: 0 10px; color: #888">--- trunk/src/wp-includes/html-api/class-wp-html-span.php                             (rev 0)
+++ trunk/src/wp-includes/html-api/class-wp-html-span.php       2023-02-03 01:03:59 UTC (rev 55203)
</span><span class="lines" style="display: block; padding: 0 10px; color: #888">@@ -0,0 +1,52 @@
</span><ins style="background-color: #dfd; text-decoration:none; display:block; padding: 0 10px">+<?php
+/**
+ * HTML Span: Represents a textual span inside an HTML document.
+ *
+ * @package WordPress
+ * @subpackage HTML-API
+ * @since 6.2.0
+ */
+
+/**
+ * Represents a textual span inside an HTML document.
+ *
+ * This is a two-tuple in disguise, used to avoid the memory
+ * overhead involved in using an array for the same purpose.
+ *
+ * This class is for internal usage of the WP_HTML_Tag_Processor class.
+ *
+ * @access private
+ * @since 6.2.0
+ *
+ * @see WP_HTML_Tag_Processor
+ */
+class WP_HTML_Span {
+       /**
+        * Byte offset into document where span begins.
+        *
+        * @since 6.2.0
+        * @var int
+        */
+       public $start;
+
+       /**
+        * Byte offset into document where span ends.
+        *
+        * @since 6.2.0
+        * @var int
+        */
+       public $end;
+
+       /**
+        * Constructor.
+        *
+        * @since 6.2.0
+        *
+        * @param int $start Byte offset into document where replacement span begins.
+        * @param int $end   Byte offset into document where replacement span ends.
+        */
+       public function __construct( $start, $end ) {
+               $this->start = $start;
+               $this->end   = $end;
+       }
+}
</ins></span></pre></div>
<a id="trunksrcwpincludeshtmlapiclasswphtmltagprocessorphp"></a>
<div class="addfile"><h4 style="background-color: #eee; color: inherit; margin: 1em 0; padding: 1.3em; font-size: 115%">Added: trunk/src/wp-includes/html-api/class-wp-html-tag-processor.php</h4>
<pre class="diff"><span>
<span class="info" style="display: block; padding: 0 10px; color: #888">--- trunk/src/wp-includes/html-api/class-wp-html-tag-processor.php                            (rev 0)
+++ trunk/src/wp-includes/html-api/class-wp-html-tag-processor.php      2023-02-03 01:03:59 UTC (rev 55203)
</span><span class="lines" style="display: block; padding: 0 10px; color: #888">@@ -0,0 +1,2264 @@
</span><ins style="background-color: #dfd; text-decoration:none; display:block; padding: 0 10px">+<?php
+/**
+ * Scans through an HTML document to find specific tags, then
+ * transforms those tags by adding, removing, or updating the
+ * values of the HTML attributes within that tag (opener).
+ *
+ * Does not fully parse HTML or _recurse_ into the HTML structure
+ * Instead this scans linearly through a document and only parses
+ * the HTML tag openers.
+ *
+ * ### Possible future direction for this module
+ *
+ *  - Prune the whitespace when removing classes/attributes: e.g. "a b c" -> "c" not " c".
+ *    This would increase the size of the changes for some operations but leave more
+ *    natural-looking output HTML.
+ *  - Decode HTML character references within class names when matching. E.g. match having
+ *    class `1<"2` needs to recognize `class="1&lt;&quot;2"`. Currently the Tag Processor
+ *    will fail to find the right tag if the class name is encoded as such.
+ *  - Properly decode HTML character references in `get_attribute()`. PHP's
+ *    `html_entity_decode()` is wrong in a couple ways: it doesn't account for the
+ *    no-ambiguous-ampersand rule, and it improperly handles the way semicolons may
+ *    or may not terminate a character reference.
+ *
+ * @package WordPress
+ * @subpackage HTML-API
+ * @since 6.2.0
+ */
+
+/**
+ * Modifies attributes in an HTML document for tags matching a query.
+ *
+ * ## Usage
+ *
+ * Use of this class requires three steps:
+ *
+ *  1. Create a new class instance with your input HTML document.
+ *  2. Find the tag(s) you are looking for.
+ *  3. Request changes to the attributes in those tag(s).
+ *
+ * Example:
+ * ```php
+ *     $tags = new WP_HTML_Tag_Processor( $html );
+ *     if ( $tags->next_tag( [ 'tag_name' => 'option' ] ) ) {
+ *         $tags->set_attribute( 'selected', true );
+ *     }
+ * ```
+ *
+ * ### Finding tags
+ *
+ * The `next_tag()` function moves the internal cursor through
+ * your input HTML document until it finds a tag meeting any of
+ * the supplied restrictions in the optional query argument. If
+ * no argument is provided then it will find the next HTML tag,
+ * regardless of what kind it is.
+ *
+ * If you want to _find whatever the next tag is_:
+ * ```php
+ *     $tags->next_tag();
+ * ```
+ *
+ * | Goal                                                      | Query                                                                      |
+ * |-----------------------------------------------------------|----------------------------------------------------------------------------|
+ * | Find any tag.                                             | `$tags->next_tag();`                                                       |
+ * | Find next image tag.                                      | `$tags->next_tag( [ 'tag_name' => 'img' ] );`                              |
+ * | Find next tag containing the `fullwidth` CSS class.       | `$tags->next_tag( [ 'class_name' => 'fullwidth' ] );`                      |
+ * | Find next image tag containing the `fullwidth` CSS class. | `$tags->next_tag( [ 'tag_name' => 'img', 'class_name' => 'fullwidth' ] );` |
+ *
+ * If a tag was found meeting your criteria then `next_tag()`
+ * will return `true` and you can proceed to modify it. If it
+ * returns `false`, however, it failed to find the tag and
+ * moved the cursor to the end of the file.
+ *
+ * Once the cursor reaches the end of the file the processor
+ * is done and if you want to reach an earlier tag you will
+ * need to recreate the processor and start over, as it's
+ * unable to back up or move in reverse.
+ *
+ * See the section on bookmarks for an exception to this
+ * no-backing-up rule.
+ *
+ * #### Custom queries
+ *
+ * Sometimes it's necessary to further inspect an HTML tag than
+ * the query syntax here permits. In these cases one may further
+ * inspect the search results using the read-only functions
+ * provided by the processor or external state or variables.
+ *
+ * Example:
+ * ```php
+ *     // Paint up to the first five DIV or SPAN tags marked with the "jazzy" style.
+ *     $remaining_count = 5;
+ *     while ( $remaining_count > 0 && $tags->next_tag() ) {
+ *         if (
+ *              ( 'DIV' === $tags->get_tag() || 'SPAN' === $tags->get_tag() ) &&
+ *              'jazzy' === $tags->get_attribute( 'data-style' )
+ *         ) {
+ *             $tags->add_class( 'theme-style-everest-jazz' );
+ *             $remaining_count--;
+ *         }
+ *     }
+ * ```
+ *
+ * `get_attribute()` will return `null` if the attribute wasn't present
+ * on the tag when it was called. It may return `""` (the empty string)
+ * in cases where the attribute was present but its value was empty.
+ * For boolean attributes, those whose name is present but no value is
+ * given, it will return `true` (the only way to set `false` for an
+ * attribute is to remove it).
+ *
+ * ### Modifying HTML attributes for a found tag
+ *
+ * Once you've found the start of an opening tag you can modify
+ * any number of the attributes on that tag. You can set a new
+ * value for an attribute, remove the entire attribute, or do
+ * nothing and move on to the next opening tag.
+ *
+ * Example:
+ * ```php
+ *     if ( $tags->next_tag( [ 'class' => 'wp-group-block' ] ) ) {
+ *         $tags->set_attribute( 'title', 'This groups the contained content.' );
+ *         $tags->remove_attribute( 'data-test-id' );
+ *     }
+ * ```
+ *
+ * If `set_attribute()` is called for an existing attribute it will
+ * overwrite the existing value. Similarly, calling `remove_attribute()`
+ * for a non-existing attribute has no effect on the document. Both
+ * of these methods are safe to call without knowing if a given attribute
+ * exists beforehand.
+ *
+ * ### Modifying CSS classes for a found tag
+ *
+ * The tag processor treats the `class` attribute as a special case.
+ * Because it's a common operation to add or remove CSS classes, this
+ * interface adds helper methods to make that easier.
+ *
+ * As with attribute values, adding or removing CSS classes is a safe
+ * operation that doesn't require checking if the attribute or class
+ * exists before making changes. If removing the only class then the
+ * entire `class` attribute will be removed.
+ *
+ * Example:
+ * ```php
+ *     // from `<span>Yippee!</span>`
+ *     //   to `<span class="is-active">Yippee!</span>`
+ *     $tags->add_class( 'is-active' );
+ *
+ *     // from `<span class="excited">Yippee!</span>`
+ *     //   to `<span class="excited is-active">Yippee!</span>`
+ *     $tags->add_class( 'is-active' );
+ *
+ *     // from `<span class="is-active heavy-accent">Yippee!</span>`
+ *     //   to `<span class="is-active heavy-accent">Yippee!</span>`
+ *     $tags->add_class( 'is-active' );
+ *
+ *     // from `<input type="text" class="is-active rugby not-disabled" length="24">`
+ *     //   to `<input type="text" class="is-active not-disabled" length="24">
+ *     $tags->remove_class( 'rugby' );
+ *
+ *     // from `<input type="text" class="rugby" length="24">`
+ *     //   to `<input type="text" length="24">
+ *     $tags->remove_class( 'rugby' );
+ *
+ *     // from `<input type="text" length="24">`
+ *     //   to `<input type="text" length="24">
+ *     $tags->remove_class( 'rugby' );
+ * ```
+ *
+ * When class changes are enqueued but a direct change to `class` is made via
+ * `set_attribute` then the changes to `set_attribute` (or `remove_attribute`)
+ * will take precedence over those made through `add_class` and `remove_class`.
+ *
+ * ### Bookmarks
+ *
+ * While scanning through the input HTMl document it's possible to set
+ * a named bookmark when a particular tag is found. Later on, after
+ * continuing to scan other tags, it's possible to `seek` to one of
+ * the set bookmarks and then proceed again from that point forward.
+ *
+ * Because bookmarks create processing overhead one should avoid
+ * creating too many of them. As a rule, create only bookmarks
+ * of known string literal names; avoid creating "mark_{$index}"
+ * and so on. It's fine from a performance standpoint to create a
+ * bookmark and update it frequently, such as within a loop.
+ *
+ * ```php
+ *     $total_todos = 0;
+ *     while ( $p->next_tag( [ 'tag_name' => 'UL', 'class_name' => 'todo' ] ) ) {
+ *         $p->set_bookmark( 'list-start' );
+ *         while ( $p->next_tag( [ 'tag_closers' => 'visit' ] ) ) {
+ *             if ( 'UL' === $p->get_tag() && $p->is_tag_closer() ) {
+ *                 $p->set_bookmark( 'list-end' );
+ *                 $p->seek( 'list-start' );
+ *                 $p->set_attribute( 'data-contained-todos', (string) $total_todos );
+ *                 $total_todos = 0;
+ *                 $p->seek( 'list-end' );
+ *                 break;
+ *             }
+ *
+ *             if ( 'LI' === $p->get_tag() && ! $p->is_tag_closer() ) {
+ *                 $total_todos++;
+ *             }
+ *         }
+ *     }
+ * ```
+ *
+ * ## Design and limitations
+ *
+ * The Tag Processor is designed to linearly scan HTML documents and tokenize
+ * HTML tags and their attributes. It's designed to do this as efficiently as
+ * possible without compromising parsing integrity. Therefore it will be
+ * slower than some methods of modifying HTML, such as those incorporating
+ * over-simplified PCRE patterns, but will not introduce the defects and
+ * failures that those methods bring in, which lead to broken page renders
+ * and often to security vulnerabilities. On the other hand, it will be faster
+ * than full-blown HTML parsers such as DOMDocument and use considerably
+ * less memory. It requires a negligible memory overhead, enough to consider
+ * it a zero-overhead system.
+ *
+ * The performance characteristics are maintained by avoiding tree construction
+ * and semantic cleanups which are specified in HTML5. Because of this, for
+ * example, it's not possible for the Tag Processor to associate any given
+ * opening tag with its corresponding closing tag, or to return the inner markup
+ * inside an element. Systems may be built on top of the Tag Processor to do
+ * this, but the Tag Processor is and should be constrained so it can remain an
+ * efficient, low-level, and reliable HTML scanner.
+ *
+ * The Tag Processor's design incorporates a "garbage-in-garbage-out" philosophy.
+ * HTML5 specifies that certain invalid content be transformed into different forms
+ * for display, such as removing null bytes from an input document and replacing
+ * invalid characters with the Unicode replacement character U+FFFD �. Where errors
+ * or transformations exist within the HTML5 specification, the Tag Processor leaves
+ * those invalid inputs untouched, passing them through to the final browser to handle.
+ * While this implies that certain operations will be non-spec-compliant, such as
+ * reading the value of an attribute with invalid content, it also preserves a
+ * simplicity and efficiency for handling those error cases.
+ *
+ * Most operations within the Tag Processor are designed to minimize the difference
+ * between an input and output document for any given change. For example, the
+ * `add_class` and `remove_class` methods preserve whitespace and the class ordering
+ * within the `class` attribute; and when encountering tags with duplicated attributes,
+ * the Tag Processor will leave those invalid duplicate attributes where they are but
+ * update the proper attribute which the browser will read for parsing its value. An
+ * exception to this rule is that all attribute updates store their values as
+ * double-quoted strings, meaning that attributes on input with single-quoted or
+ * unquoted values will appear in the output with double-quotes.
+ *
+ * @since 6.2.0
+ */
+class WP_HTML_Tag_Processor {
+       /**
+        * The maximum number of bookmarks allowed to exist at
+        * any given time.
+        *
+        * @see set_bookmark()
+        * @since 6.2.0
+        * @var int
+        */
+       const MAX_BOOKMARKS = 10;
+
+       /**
+        * Maximum number of times seek() can be called.
+        * Prevents accidental infinite loops.
+        *
+        * @see seek()
+        * @since 6.2.0
+        * @var int
+        */
+       const MAX_SEEK_OPS = 1000;
+
+       /**
+        * The HTML document to parse.
+        *
+        * @since 6.2.0
+        * @var string
+        */
+       private $html;
+
+       /**
+        * The last query passed to next_tag().
+        *
+        * @since 6.2.0
+        * @var array|null
+        */
+       private $last_query;
+
+       /**
+        * The tag name this processor currently scans for.
+        *
+        * @since 6.2.0
+        * @var string|null
+        */
+       private $sought_tag_name;
+
+       /**
+        * The CSS class name this processor currently scans for.
+        *
+        * @since 6.2.0
+        * @var string|null
+        */
+       private $sought_class_name;
+
+       /**
+        * The match offset this processor currently scans for.
+        *
+        * @since 6.2.0
+        * @var int|null
+        */
+       private $sought_match_offset;
+
+       /**
+        * Whether to visit tag closers, e.g. </div>, when walking an input document.
+        *
+        * @since 6.2.0
+        * @var bool
+        */
+       private $stop_on_tag_closers;
+
+       /**
+        * Holds updated HTML as updates are applied.
+        *
+        * Updates and unmodified portions of the input document are
+        * appended to this value as they are applied. It will hold
+        * a copy of the updated document up until the point of the
+        * latest applied update. The fully-updated HTML document
+        * will comprise this value plus the part of the input document
+        * which follows that latest update.
+        *
+        * @see $bytes_already_copied
+        *
+        * @since 6.2.0
+        * @var string
+        */
+       private $output_buffer = '';
+
+       /**
+        * How many bytes from the original HTML document have been read and parsed.
+        *
+        * This value points to the latest byte offset in the input document which
+        * has been already parsed. It is the internal cursor for the Tag Processor
+        * and updates while scanning through the HTML tokens.
+        *
+        * @since 6.2.0
+        * @var int
+        */
+       private $bytes_already_parsed = 0;
+
+       /**
+        * How many bytes from the input HTML document have already been
+        * copied into the output buffer.
+        *
+        * Lexical updates are enqueued and processed in batches. Prior
+        * to any given update in the input document, there might exist
+        * a span of HTML unaffected by any changes. This span ought to
+        * be copied verbatim into the output buffer before applying the
+        * following update. This value will point to the starting byte
+        * offset in the input document where that unaffected span of
+        * HTML starts.
+        *
+        * @since 6.2.0
+        * @var int
+        */
+       private $bytes_already_copied = 0;
+
+       /**
+        * Byte offset in input document where current tag name starts.
+        *
+        * Example:
+        * ```
+        *   <div id="test">...
+        *   01234
+        *    - tag name starts at 1
+        * ```
+        *
+        * @since 6.2.0
+        * @var int|null
+        */
+       private $tag_name_starts_at;
+
+       /**
+        * Byte length of current tag name.
+        *
+        * Example:
+        * ```
+        *   <div id="test">...
+        *   01234
+        *    --- tag name length is 3
+        * ```
+        *
+        * @since 6.2.0
+        * @var int|null
+        */
+       private $tag_name_length;
+
+       /**
+        * Byte offset in input document where current tag token ends.
+        *
+        * Example:
+        * ```
+        *   <div id="test">...
+        *   0         1   |
+        *   01234567890123456
+        *    --- tag name ends at 14
+        * ```
+        *
+        * @since 6.2.0
+        * @var int|null
+        */
+       private $tag_ends_at;
+
+       /**
+        * Whether the current tag is an opening tag, e.g. <div>, or a closing tag, e.g. </div>.
+        *
+        * @var bool
+        */
+       private $is_closing_tag;
+
+       /**
+        * Lazily-built index of attributes found within an HTML tag, keyed by the attribute name.
+        *
+        * Example:
+        * ```php
+        *     // supposing the parser is working through this content
+        *     // and stops after recognizing the `id` attribute
+        *     // <div id="test-4" class=outline title="data:text/plain;base64=asdk3nk1j3fo8">
+        *     //                 ^ parsing will continue from this point
+        *     $this->attributes = [
+        *         'id' => new WP_HTML_Attribute_Match( 'id', null, 6, 17 )
+        *     ];
+        *
+        *     // when picking up parsing again, or when asking to find the
+        *     // `class` attribute we will continue and add to this array
+        *     $this->attributes = [
+        *         'id'    => new WP_HTML_Attribute_Match( 'id', null, 6, 17 ),
+        *         'class' => new WP_HTML_Attribute_Match( 'class', 'outline', 18, 32 )
+        *     ];
+        *
+        *     // Note that only the `class` attribute value is stored in the index.
+        *     // That's because it is the only value used by this class at the moment.
+        * ```
+        *
+        * @since 6.2.0
+        * @var WP_HTML_Attribute_Token[]
+        */
+       private $attributes = array();
+
+       /**
+        * Which class names to add or remove from a tag.
+        *
+        * These are tracked separately from attribute updates because they are
+        * semantically distinct, whereas this interface exists for the common
+        * case of adding and removing class names while other attributes are
+        * generally modified as with DOM `setAttribute` calls.
+        *
+        * When modifying an HTML document these will eventually be collapsed
+        * into a single `set_attribute( 'class', $changes )` call.
+        *
+        * Example:
+        * ```php
+        *     // Add the `wp-block-group` class, remove the `wp-group` class.
+        *     $classname_updates = [
+        *         // Indexed by a comparable class name
+        *         'wp-block-group' => WP_HTML_Tag_Processor::ADD_CLASS,
+        *         'wp-group'       => WP_HTML_Tag_Processor::REMOVE_CLASS
+        *     ];
+        * ```
+        *
+        * @since 6.2.0
+        * @var bool[]
+        */
+       private $classname_updates = array();
+
+       /**
+        * Tracks a semantic location in the original HTML which
+        * shifts with updates as they are applied to the document.
+        *
+        * @since 6.2.0
+        * @var WP_HTML_Span[]
+        */
+       protected $bookmarks = array();
+
+       const ADD_CLASS    = true;
+       const REMOVE_CLASS = false;
+       const SKIP_CLASS   = null;
+
+       /**
+        * Lexical replacements to apply to input HTML document.
+        *
+        * "Lexical" in this class refers to the part of this class which
+        * operates on pure text _as text_ and not as HTML. There's a line
+        * between the public interface, with HTML-semantic methods like
+        * `set_attribute` and `add_class`, and an internal state that tracks
+        * text offsets in the input document.
+        *
+        * When higher-level HTML methods are called, those have to transform their
+        * operations (such as setting an attribute's value) into text diffing
+        * operations (such as replacing the sub-string from indices A to B with
+        * some given new string). These text-diffing operations are the lexical
+        * updates.
+        *
+        * As new higher-level methods are added they need to collapse their
+        * operations into these lower-level lexical updates since that's the
+        * Tag Processor's internal language of change. Any code which creates
+        * these lexical updates must ensure that they do not cross HTML syntax
+        * boundaries, however, so these should never be exposed outside of this
+        * class or any classes which intentionally expand its functionality.
+        *
+        * These are enqueued while editing the document instead of being immediately
+        * applied to avoid processing overhead, string allocations, and string
+        * copies when applying many updates to a single document.
+        *
+        * Example:
+        * ```php
+        *     // Replace an attribute stored with a new value, indices
+        *     // sourced from the lazily-parsed HTML recognizer.
+        *     $start = $attributes['src']->start;
+        *     $end   = $attributes['src']->end;
+        *     $modifications[] = new WP_HTML_Text_Replacement( $start, $end, $new_value );
+        *
+        *     // Correspondingly, something like this will appear in this array.
+        *     $lexical_updates = [
+        *         WP_HTML_Text_Replacement( 14, 28, 'https://my-site.my-domain/wp-content/uploads/2014/08/kittens.jpg' )
+        *     ];
+        * ```
+        *
+        * @since 6.2.0
+        * @var WP_HTML_Text_Replacement[]
+        */
+       protected $lexical_updates = array();
+
+       /**
+        * Tracks and limits `seek()` calls to prevent accidental infinite loops.
+        *
+        * @see seek
+        * @since 6.2.0
+        * @var int
+        */
+       protected $seek_count = 0;
+
+       /**
+        * Constructor.
+        *
+        * @since 6.2.0
+        *
+        * @param string $html HTML to process.
+        */
+       public function __construct( $html ) {
+               $this->html = $html;
+       }
+
+       /**
+        * Finds the next tag matching the $query.
+        *
+        * @since 6.2.0
+        *
+        * @param array|string|null $query {
+        *     Optional. Which tag name to find, having which class, etc. Default is to find any tag.
+        *
+        *     @type string|null $tag_name     Which tag to find, or `null` for "any tag."
+        *     @type int|null    $match_offset Find the Nth tag matching all search criteria.
+        *                                     0 for "first" tag, 2 for "third," etc.
+        *                                     Defaults to first tag.
+        *     @type string|null $class_name   Tag must contain this whole class name to match.
+        *     @type string|null $tag_closers  "visit" or "skip": whether to stop on tag closers, e.g. </div>.
+        * }
+        * @return boolean Whether a tag was matched.
+        */
+       public function next_tag( $query = null ) {
+               $this->parse_query( $query );
+               $already_found = 0;
+
+               do {
+                       if ( $this->bytes_already_parsed >= strlen( $this->html ) ) {
+                               return false;
+                       }
+
+                       // Find the next tag if it exists.
+                       if ( false === $this->parse_next_tag() ) {
+                               $this->bytes_already_parsed = strlen( $this->html );
+
+                               return false;
+                       }
+
+                       // Parse all of its attributes.
+                       while ( $this->parse_next_attribute() ) {
+                               continue;
+                       }
+
+                       // Ensure that the tag closes before the end of the document.
+                       $tag_ends_at = strpos( $this->html, '>', $this->bytes_already_parsed );
+                       if ( false === $tag_ends_at ) {
+                               return false;
+                       }
+                       $this->tag_ends_at          = $tag_ends_at;
+                       $this->bytes_already_parsed = $tag_ends_at;
+
+                       // Finally, check if the parsed tag and its attributes match the search query.
+                       if ( $this->matches() ) {
+                               ++$already_found;
+                       }
+
+                       /*
+                        * For non-DATA sections which might contain text that looks like HTML tags but
+                        * isn't, scan with the appropriate alternative mode. Looking at the first letter
+                        * of the tag name as a pre-check avoids a string allocation when it's not needed.
+                        */
+                       $t = $this->html[ $this->tag_name_starts_at ];
+                       if ( ! $this->is_closing_tag && ( 's' === $t || 'S' === $t || 't' === $t || 'T' === $t ) ) {
+                               $tag_name = $this->get_tag();
+
+                               if ( 'SCRIPT' === $tag_name && ! $this->skip_script_data() ) {
+                                       $this->bytes_already_parsed = strlen( $this->html );
+                                       return false;
+                               } elseif (
+                                       ( 'TEXTAREA' === $tag_name || 'TITLE' === $tag_name ) &&
+                                       ! $this->skip_rcdata( $tag_name )
+                               ) {
+                                       $this->bytes_already_parsed = strlen( $this->html );
+                                       return false;
+                               }
+                       }
+               } while ( $already_found < $this->sought_match_offset );
+
+               return true;
+       }
+
+
+       /**
+        * Sets a bookmark in the HTML document.
+        *
+        * Bookmarks represent specific places or tokens in the HTML
+        * document, such as a tag opener or closer. When applying
+        * edits to a document, such as setting an attribute, the
+        * text offsets of that token may shift; the bookmark is
+        * kept updated with those shifts and remains stable unless
+        * the entire span of text in which the token sits is removed.
+        *
+        * Release bookmarks when they are no longer needed.
+        *
+        * Example:
+        * ```
+        *     <main><h2>Surprising fact you may not know!</h2></main>
+        *           ^  ^
+        *            \-|-- this `H2` opener bookmark tracks the token
+        *
+        *     <main class="clickbait"><h2>Surprising fact you may no…
+        *                             ^  ^
+        *                              \-|-- it shifts with edits
+        * ```
+        *
+        * Bookmarks provide the ability to seek to a previously-scanned
+        * place in the HTML document. This avoids the need to re-scan
+        * the entire document.
+        *
+        * Example:
+        * ```
+        *     <ul><li>One</li><li>Two</li><li>Three</li></ul>
+        *                                 ^^^^
+        *                                 want to note this last item
+        *
+        *     $p = new WP_HTML_Tag_Processor( $html );
+        *     $in_list = false;
+        *     while ( $p->next_tag( [ 'tag_closers' => $in_list ? 'visit' : 'skip' ] ) ) {
+        *         if ( 'UL' === $p->get_tag() ) {
+        *             if ( $p->is_tag_closer() ) {
+        *                 $in_list = false;
+        *                 $p->set_bookmark( 'resume' );
+        *                 if ( $p->seek( 'last-li' ) ) {
+        *                     $p->add_class( 'last-li' );
+        *                 }
+        *                 $p->seek( 'resume' );
+        *                 $p->release_bookmark( 'last-li' );
+        *                 $p->release_bookmark( 'resume' );
+        *             } else {
+        *                 $in_list = true;
+        *             }
+        *         }
+        *
+        *         if ( 'LI' === $p->get_tag() ) {
+        *             $p->set_bookmark( 'last-li' );
+        *         }
+        *     }
+        * ```
+        *
+        * Bookmarks intentionally hide the internal string offsets
+        * to which they refer. They are maintained internally as
+        * updates are applied to the HTML document and therefore
+        * retain their "position" - the location to which they
+        * originally pointed. The inability to use bookmarks with
+        * functions like `substr` is therefore intentional to guard
+        * against accidentally breaking the HTML.
+        *
+        * Because bookmarks allocate memory and require processing
+        * for every applied update, they are limited and require
+        * a name. They should not be created with programmatically-made
+        * names, such as "li_{$index}" with some loop. As a general
+        * rule they should only be created with string-literal names
+        * like "start-of-section" or "last-paragraph".
+        *
+        * Bookmarks are a powerful tool to enable complicated behavior.
+        * Consider double-checking that you need this tool if you are
+        * reaching for it, as inappropriate use could lead to broken
+        * HTML structure or unwanted processing overhead.
+        *
+        * @since 6.2.0
+        *
+        * @param string $name Identifies this particular bookmark.
+        * @return bool Whether the bookmark was successfully created.
+        */
+       public function set_bookmark( $name ) {
+               if ( null === $this->tag_name_starts_at ) {
+                       return false;
+               }
+
+               if ( ! array_key_exists( $name, $this->bookmarks ) && count( $this->bookmarks ) >= self::MAX_BOOKMARKS ) {
+                       _doing_it_wrong(
+                               __METHOD__,
+                               __( 'Too many bookmarks: cannot create any more.' ),
+                               '6.2.0'
+                       );
+                       return false;
+               }
+
+               $this->bookmarks[ $name ] = new WP_HTML_Span(
+                       $this->tag_name_starts_at - 1,
+                       $this->tag_ends_at
+               );
+
+               return true;
+       }
+
+
+       /**
+        * Removes a bookmark that is no longer needed.
+        *
+        * Releasing a bookmark frees up the small
+        * performance overhead it requires.
+        *
+        * @param string $name Name of the bookmark to remove.
+        * @return bool Whether the bookmark already existed before removal.
+        */
+       public function release_bookmark( $name ) {
+               if ( ! array_key_exists( $name, $this->bookmarks ) ) {
+                       return false;
+               }
+
+               unset( $this->bookmarks[ $name ] );
+
+               return true;
+       }
+
+
+       /**
+        * Skips contents of title and textarea tags.
+        *
+        * @see https://html.spec.whatwg.org/multipage/parsing.html#rcdata-state
+        * @since 6.2.0
+        *
+        * @param string $tag_name – the lowercase tag name which will close the RCDATA region.
+        * @return bool Whether an end to the RCDATA region was found before the end of the document.
+        */
+       private function skip_rcdata( $tag_name ) {
+               $html       = $this->html;
+               $doc_length = strlen( $html );
+               $tag_length = strlen( $tag_name );
+
+               $at = $this->bytes_already_parsed;
+
+               while ( false !== $at && $at < $doc_length ) {
+                       $at = strpos( $this->html, '</', $at );
+
+                       // If there is no possible tag closer then fail.
+                       if ( false === $at || ( $at + $tag_length ) >= $doc_length ) {
+                               $this->bytes_already_parsed = $doc_length;
+                               return false;
+                       }
+
+                       $at += 2;
+
+                       /*
+                        * Find a case-insensitive match to the tag name.
+                        *
+                        * Because tag names are limited to US-ASCII there is no
+                        * need to perform any kind of Unicode normalization when
+                        * comparing; any character which could be impacted by such
+                        * normalization could not be part of a tag name.
+                        */
+                       for ( $i = 0; $i < $tag_length; $i++ ) {
+                               $tag_char  = $tag_name[ $i ];
+                               $html_char = $html[ $at + $i ];
+
+                               if ( $html_char !== $tag_char && strtoupper( $html_char ) !== $tag_char ) {
+                                       $at += $i;
+                                       continue 2;
+                               }
+                       }
+
+                       $at                        += $tag_length;
+                       $this->bytes_already_parsed = $at;
+
+                       /*
+                        * Ensure that the tag name terminates to avoid matching on
+                        * substrings of a longer tag name. For example, the sequence
+                        * "</textarearug" should not match for "</textarea" even
+                        * though "textarea" is found within the text.
+                        */
+                       $c = $html[ $at ];
+                       if ( ' ' !== $c && "\t" !== $c && "\r" !== $c && "\n" !== $c && '/' !== $c && '>' !== $c ) {
+                               continue;
+                       }
+
+                       while ( $this->parse_next_attribute() ) {
+                               continue;
+                       }
+                       $at = $this->bytes_already_parsed;
+                       if ( $at >= strlen( $this->html ) ) {
+                               return false;
+                       }
+
+                       if ( '>' === $html[ $at ] || '/' === $html[ $at ] ) {
+                               ++$this->bytes_already_parsed;
+                               return true;
+                       }
+               }
+
+               return false;
+       }
+
+       /**
+        * Skips contents of script tags.
+        *
+        * @since 6.2.0
+        *
+        * @return bool Whether the script tag was closed before the end of the document.
+        */
+       private function skip_script_data() {
+               $state      = 'unescaped';
+               $html       = $this->html;
+               $doc_length = strlen( $html );
+               $at         = $this->bytes_already_parsed;
+
+               while ( false !== $at && $at < $doc_length ) {
+                       $at += strcspn( $html, '-<', $at );
+
+                       /*
+                        * For all script states a "-->"  transitions
+                        * back into the normal unescaped script mode,
+                        * even if that's the current state.
+                        */
+                       if (
+                               $at + 2 < $doc_length &&
+                               '-' === $html[ $at ] &&
+                               '-' === $html[ $at + 1 ] &&
+                               '>' === $html[ $at + 2 ]
+                       ) {
+                               $at   += 3;
+                               $state = 'unescaped';
+                               continue;
+                       }
+
+                       // Everything of interest past here starts with "<".
+                       if ( $at + 1 >= $doc_length || '<' !== $html[ $at++ ] ) {
+                               continue;
+                       }
+
+                       /*
+                        * Unlike with "-->", the "<!--" only transitions
+                        * into the escaped mode if not already there.
+                        *
+                        * Inside the escaped modes it will be ignored; and
+                        * should never break out of the double-escaped
+                        * mode and back into the escaped mode.
+                        *
+                        * While this requires a mode change, it does not
+                        * impact the parsing otherwise, so continue
+                        * parsing after updating the state.
+                        */
+                       if (
+                               $at + 2 < $doc_length &&
+                               '!' === $html[ $at ] &&
+                               '-' === $html[ $at + 1 ] &&
+                               '-' === $html[ $at + 2 ]
+                       ) {
+                               $at   += 3;
+                               $state = 'unescaped' === $state ? 'escaped' : $state;
+                               continue;
+                       }
+
+                       if ( '/' === $html[ $at ] ) {
+                               $is_closing = true;
+                               ++$at;
+                       } else {
+                               $is_closing = false;
+                       }
+
+                       /*
+                        * At this point the only remaining state-changes occur with the
+                        * <script> and </script> tags; unless one of these appears next,
+                        * proceed scanning to the next potential token in the text.
+                        */
+                       if ( ! (
+                               $at + 6 < $doc_length &&
+                               ( 's' === $html[ $at ] || 'S' === $html[ $at ] ) &&
+                               ( 'c' === $html[ $at + 1 ] || 'C' === $html[ $at + 1 ] ) &&
+                               ( 'r' === $html[ $at + 2 ] || 'R' === $html[ $at + 2 ] ) &&
+                               ( 'i' === $html[ $at + 3 ] || 'I' === $html[ $at + 3 ] ) &&
+                               ( 'p' === $html[ $at + 4 ] || 'P' === $html[ $at + 4 ] ) &&
+                               ( 't' === $html[ $at + 5 ] || 'T' === $html[ $at + 5 ] )
+                       ) ) {
+                               ++$at;
+                               continue;
+                       }
+
+                       /*
+                        * Ensure that the script tag terminates to avoid matching on
+                        * substrings of a non-match. For example, the sequence
+                        * "<script123" should not end a script region even though
+                        * "<script" is found within the text.
+                        */
+                       if ( $at + 6 >= $doc_length ) {
+                               continue;
+                       }
+                       $at += 6;
+                       $c   = $html[ $at ];
+                       if ( ' ' !== $c && "\t" !== $c && "\r" !== $c && "\n" !== $c && '/' !== $c && '>' !== $c ) {
+                               ++$at;
+                               continue;
+                       }
+
+                       if ( 'escaped' === $state && ! $is_closing ) {
+                               $state = 'double-escaped';
+                               continue;
+                       }
+
+                       if ( 'double-escaped' === $state && $is_closing ) {
+                               $state = 'escaped';
+                               continue;
+                       }
+
+                       if ( $is_closing ) {
+                               $this->bytes_already_parsed = $at;
+                               if ( $this->bytes_already_parsed >= $doc_length ) {
+                                       return false;
+                               }
+
+                               while ( $this->parse_next_attribute() ) {
+                                       continue;
+                               }
+
+                               if ( '>' === $html[ $this->bytes_already_parsed ] ) {
+                                       ++$this->bytes_already_parsed;
+                                       return true;
+                               }
+                       }
+
+                       ++$at;
+               }
+
+               return false;
+       }
+
+       /**
+        * Parses the next tag.
+        *
+        * This will find and start parsing the next tag, including
+        * the opening `<`, the potential closer `/`, and the tag
+        * name. It does not parse the attributes or scan to the
+        * closing `>`; these are left for other methods.
+        *
+        * @since 6.2.0
+        *
+        * @return bool Whether a tag was found before the end of the document.
+        */
+       private function parse_next_tag() {
+               $this->after_tag();
+
+               $html       = $this->html;
+               $doc_length = strlen( $html );
+               $at         = $this->bytes_already_parsed;
+
+               while ( false !== $at && $at < $doc_length ) {
+                       $at = strpos( $html, '<', $at );
+                       if ( false === $at ) {
+                               return false;
+                       }
+
+                       if ( '/' === $this->html[ $at + 1 ] ) {
+                               $this->is_closing_tag = true;
+                               $at++;
+                       } else {
+                               $this->is_closing_tag = false;
+                       }
+
+                       /*
+                        * HTML tag names must start with [a-zA-Z] otherwise they are not tags.
+                        * For example, "<3" is rendered as text, not a tag opener. If at least
+                        * one letter follows the "<" then _it is_ a tag, but if the following
+                        * character is anything else it _is not a tag_.
+                        *
+                        * It's not uncommon to find non-tags starting with `<` in an HTML
+                        * document, so it's good for performance to make this pre-check before
+                        * continuing to attempt to parse a tag name.
+                        *
+                        * Reference:
+                        * * https://html.spec.whatwg.org/multipage/parsing.html#data-state
+                        * * https://html.spec.whatwg.org/multipage/parsing.html#tag-open-state
+                        */
+                       $tag_name_prefix_length = strspn( $html, 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ', $at + 1 );
+                       if ( $tag_name_prefix_length > 0 ) {
+                               ++$at;
+                               $this->tag_name_length      = $tag_name_prefix_length + strcspn( $html, " \t\f\r\n/>", $at + $tag_name_prefix_length );
+                               $this->tag_name_starts_at   = $at;
+                               $this->bytes_already_parsed = $at + $this->tag_name_length;
+                               return true;
+                       }
+
+                       /*
+                        * Abort if no tag is found before the end of
+                        * the document. There is nothing left to parse.
+                        */
+                       if ( $at + 1 >= strlen( $html ) ) {
+                               return false;
+                       }
+
+                       /*
+                        * <! transitions to markup declaration open state
+                        * https://html.spec.whatwg.org/multipage/parsing.html#markup-declaration-open-state
+                        */
+                       if ( '!' === $html[ $at + 1 ] ) {
+                               /*
+                                * <!-- transitions to a bogus comment state – skip to the nearest -->
+                                * https://html.spec.whatwg.org/multipage/parsing.html#tag-open-state
+                                */
+                               if (
+                                       strlen( $html ) > $at + 3 &&
+                                       '-' === $html[ $at + 2 ] &&
+                                       '-' === $html[ $at + 3 ]
+                               ) {
+                                       $closer_at = strpos( $html, '-->', $at + 4 );
+                                       if ( false === $closer_at ) {
+                                               return false;
+                                       }
+
+                                       $at = $closer_at + 3;
+                                       continue;
+                               }
+
+                               /*
+                                * <![CDATA[ transitions to CDATA section state – skip to the nearest ]]>
+                                * The CDATA is case-sensitive.
+                                * https://html.spec.whatwg.org/multipage/parsing.html#tag-open-state
+                                */
+                               if (
+                                       strlen( $html ) > $at + 8 &&
+                                       '[' === $html[ $at + 2 ] &&
+                                       'C' === $html[ $at + 3 ] &&
+                                       'D' === $html[ $at + 4 ] &&
+                                       'A' === $html[ $at + 5 ] &&
+                                       'T' === $html[ $at + 6 ] &&
+                                       'A' === $html[ $at + 7 ] &&
+                                       '[' === $html[ $at + 8 ]
+                               ) {
+                                       $closer_at = strpos( $html, ']]>', $at + 9 );
+                                       if ( false === $closer_at ) {
+                                               return false;
+                                       }
+
+                                       $at = $closer_at + 3;
+                                       continue;
+                               }
+
+                               /*
+                                * <!DOCTYPE transitions to DOCTYPE state – skip to the nearest >
+                                * These are ASCII-case-insensitive.
+                                * https://html.spec.whatwg.org/multipage/parsing.html#tag-open-state
+                                */
+                               if (
+                                       strlen( $html ) > $at + 8 &&
+                                       ( 'D' === $html[ $at + 2 ] || 'd' === $html[ $at + 2 ] ) &&
+                                       ( 'O' === $html[ $at + 3 ] || 'o' === $html[ $at + 3 ] ) &&
+                                       ( 'C' === $html[ $at + 4 ] || 'c' === $html[ $at + 4 ] ) &&
+                                       ( 'T' === $html[ $at + 5 ] || 't' === $html[ $at + 5 ] ) &&
+                                       ( 'Y' === $html[ $at + 6 ] || 'y' === $html[ $at + 6 ] ) &&
+                                       ( 'P' === $html[ $at + 7 ] || 'p' === $html[ $at + 7 ] ) &&
+                                       ( 'E' === $html[ $at + 8 ] || 'e' === $html[ $at + 8 ] )
+                               ) {
+                                       $closer_at = strpos( $html, '>', $at + 9 );
+                                       if ( false === $closer_at ) {
+                                               return false;
+                                       }
+
+                                       $at = $closer_at + 1;
+                                       continue;
+                               }
+
+                               /*
+                                * Anything else here is an incorrectly-opened comment and transitions
+                                * to the bogus comment state - skip to the nearest >.
+                                */
+                               $at = strpos( $html, '>', $at + 1 );
+                               continue;
+                       }
+
+                       /*
+                        * <? transitions to a bogus comment state – skip to the nearest >
+                        * https://html.spec.whatwg.org/multipage/parsing.html#tag-open-state
+                        */
+                       if ( '?' === $html[ $at + 1 ] ) {
+                               $closer_at = strpos( $html, '>', $at + 2 );
+                               if ( false === $closer_at ) {
+                                       return false;
+                               }
+
+                               $at = $closer_at + 1;
+                               continue;
+                       }
+
+                       ++$at;
+               }
+
+               return false;
+       }
+
+       /**
+        * Parses the next attribute.
+        *
+        * @since 6.2.0
+        *
+        * @return bool Whether an attribute was found before the end of the document.
+        */
+       private function parse_next_attribute() {
+               // Skip whitespace and slashes.
+               $this->bytes_already_parsed += strspn( $this->html, " \t\f\r\n/", $this->bytes_already_parsed );
+               if ( $this->bytes_already_parsed >= strlen( $this->html ) ) {
+                       return false;
+               }
+
+               /*
+                * Treat the equal sign as a part of the attribute
+                * name if it is the first encountered byte.
+                *
+                * @see https://html.spec.whatwg.org/multipage/parsing.html#before-attribute-name-state
+                */
+               $name_length = '=' === $this->html[ $this->bytes_already_parsed ]
+                       ? 1 + strcspn( $this->html, "=/> \t\f\r\n", $this->bytes_already_parsed + 1 )
+                       : strcspn( $this->html, "=/> \t\f\r\n", $this->bytes_already_parsed );
+
+               // No attribute, just tag closer.
+               if ( 0 === $name_length || $this->bytes_already_parsed + $name_length >= strlen( $this->html ) ) {
+                       return false;
+               }
+
+               $attribute_start             = $this->bytes_already_parsed;
+               $attribute_name              = substr( $this->html, $attribute_start, $name_length );
+               $this->bytes_already_parsed += $name_length;
+               if ( $this->bytes_already_parsed >= strlen( $this->html ) ) {
+                       return false;
+               }
+
+               $this->skip_whitespace();
+               if ( $this->bytes_already_parsed >= strlen( $this->html ) ) {
+                       return false;
+               }
+
+               $has_value = '=' === $this->html[ $this->bytes_already_parsed ];
+               if ( $has_value ) {
+                       ++$this->bytes_already_parsed;
+                       $this->skip_whitespace();
+                       if ( $this->bytes_already_parsed >= strlen( $this->html ) ) {
+                               return false;
+                       }
+
+                       switch ( $this->html[ $this->bytes_already_parsed ] ) {
+                               case "'":
+                               case '"':
+                                       $quote                      = $this->html[ $this->bytes_already_parsed ];
+                                       $value_start                = $this->bytes_already_parsed + 1;
+                                       $value_length               = strcspn( $this->html, $quote, $value_start );
+                                       $attribute_end              = $value_start + $value_length + 1;
+                                       $this->bytes_already_parsed = $attribute_end;
+                                       break;
+
+                               default:
+                                       $value_start                = $this->bytes_already_parsed;
+                                       $value_length               = strcspn( $this->html, "> \t\f\r\n", $value_start );
+                                       $attribute_end              = $value_start + $value_length;
+                                       $this->bytes_already_parsed = $attribute_end;
+                       }
+               } else {
+                       $value_start   = $this->bytes_already_parsed;
+                       $value_length  = 0;
+                       $attribute_end = $attribute_start + $name_length;
+               }
+
+               if ( $attribute_end >= strlen( $this->html ) ) {
+                       return false;
+               }
+
+               if ( $this->is_closing_tag ) {
+                       return true;
+               }
+
+               /*
+                * > There must never be two or more attributes on
+                * > the same start tag whose names are an ASCII
+                * > case-insensitive match for each other.
+                *     - HTML 5 spec
+                *
+                * @see https://html.spec.whatwg.org/multipage/syntax.html#attributes-2:ascii-case-insensitive
+                */
+               $comparable_name = strtolower( $attribute_name );
+
+               // If an attribute is listed many times, only use the first declaration and ignore the rest.
+               if ( ! array_key_exists( $comparable_name, $this->attributes ) ) {
+                       $this->attributes[ $comparable_name ] = new WP_HTML_Attribute_Token(
+                               $attribute_name,
+                               $value_start,
+                               $value_length,
+                               $attribute_start,
+                               $attribute_end,
+                               ! $has_value
+                       );
+               }
+
+               return true;
+       }
+
+       /**
+        * Move the internal cursor past any immediate successive whitespace.
+        *
+        * @since 6.2.0
+        *
+        * @return void
+        */
+       private function skip_whitespace() {
+               $this->bytes_already_parsed += strspn( $this->html, " \t\f\r\n", $this->bytes_already_parsed );
+       }
+
+       /**
+        * Applies attribute updates and cleans up once a tag is fully parsed.
+        *
+        * @since 6.2.0
+        *
+        * @return void
+        */
+       private function after_tag() {
+               $this->class_name_updates_to_attributes_updates();
+               $this->apply_attributes_updates();
+               $this->tag_name_starts_at = null;
+               $this->tag_name_length    = null;
+               $this->tag_ends_at        = null;
+               $this->is_closing_tag     = null;
+               $this->attributes         = array();
+       }
+
+       /**
+        * Converts class name updates into tag attributes updates
+        * (they are accumulated in different data formats for performance).
+        *
+        * @see $lexical_updates
+        * @see $classname_updates
+        *
+        * @since 6.2.0
+        *
+        * @return void
+        */
+       private function class_name_updates_to_attributes_updates() {
+               if ( count( $this->classname_updates ) === 0 ) {
+                       return;
+               }
+
+               $existing_class = $this->get_enqueued_attribute_value( 'class' );
+               if ( null === $existing_class || true === $existing_class ) {
+                       $existing_class = '';
+               }
+
+               if ( false === $existing_class && isset( $this->attributes['class'] ) ) {
+                       $existing_class = substr(
+                               $this->html,
+                               $this->attributes['class']->value_starts_at,
+                               $this->attributes['class']->value_length
+                       );
+               }
+
+               if ( false === $existing_class ) {
+                       $existing_class = '';
+               }
+
+               /**
+                * Updated "class" attribute value.
+                *
+                * This is incrementally built while scanning through the existing class
+                * attribute, skipping removed classes on the way, and then appending
+                * added classes at the end. Only when finished processing will the
+                * value contain the final new value.
+
+                * @var string $class
+                */
+               $class = '';
+
+               /**
+                * Tracks the cursor position in the existing
+                * class attribute value while parsing.
+                *
+                * @var int $at
+                */
+               $at = 0;
+
+               /**
+                * Indicates if there's any need to modify the existing class attribute.
+                *
+                * If a call to `add_class()` and `remove_class()` wouldn't impact
+                * the `class` attribute value then there's no need to rebuild it.
+                * For example, when adding a class that's already present or
+                * removing one that isn't.
+                *
+                * This flag enables a performance optimization when none of the enqueued
+                * class updates would impact the `class` attribute; namely, that the
+                * processor can continue without modifying the input document, as if
+                * none of the `add_class()` or `remove_class()` calls had been made.
+                *
+                * This flag is set upon the first change that requires a string update.
+                *
+                * @var bool $modified
+                */
+               $modified = false;
+
+               // Remove unwanted classes by only copying the new ones.
+               $existing_class_length = strlen( $existing_class );
+               while ( $at < $existing_class_length ) {
+                       // Skip to the first non-whitespace character.
+                       $ws_at     = $at;
+                       $ws_length = strspn( $existing_class, " \t\f\r\n", $ws_at );
+                       $at       += $ws_length;
+
+                       // Capture the class name – it's everything until the next whitespace.
+                       $name_length = strcspn( $existing_class, " \t\f\r\n", $at );
+                       if ( 0 === $name_length ) {
+                               // If no more class names are found then that's the end.
+                               break;
+                       }
+
+                       $name = substr( $existing_class, $at, $name_length );
+                       $at  += $name_length;
+
+                       // If this class is marked for removal, start processing the next one.
+                       $remove_class = (
+                               isset( $this->classname_updates[ $name ] ) &&
+                               self::REMOVE_CLASS === $this->classname_updates[ $name ]
+                       );
+
+                       // If a class has already been seen then skip it; it should not be added twice.
+                       if ( ! $remove_class ) {
+                               $this->classname_updates[ $name ] = self::SKIP_CLASS;
+                       }
+
+                       if ( $remove_class ) {
+                               $modified = true;
+                               continue;
+                       }
+
+                       /*
+                        * Otherwise, append it to the new "class" attribute value.
+                        *
+                        * There are options for handling whitespace between tags.
+                        * Preserving the existing whitespace produces fewer changes
+                        * to the HTML content and should clarify the before/after
+                        * content when debugging the modified output.
+                        *
+                        * This approach contrasts normalizing the inter-class
+                        * whitespace to a single space, which might appear cleaner
+                        * in the output HTML but produce a noisier change.
+                        */
+                       $class .= substr( $existing_class, $ws_at, $ws_length );
+                       $class .= $name;
+               }
+
+               // Add new classes by appending those which haven't already been seen.
+               foreach ( $this->classname_updates as $name => $operation ) {
+                       if ( self::ADD_CLASS === $operation ) {
+                               $modified = true;
+
+                               $class .= strlen( $class ) > 0 ? ' ' : '';
+                               $class .= $name;
+                       }
+               }
+
+               $this->classname_updates = array();
+               if ( ! $modified ) {
+                       return;
+               }
+
+               if ( strlen( $class ) > 0 ) {
+                       $this->set_attribute( 'class', $class );
+               } else {
+                       $this->remove_attribute( 'class' );
+               }
+       }
+
+       /**
+        * Applies attribute updates to HTML document.
+        *
+        * @since 6.2.0
+        *
+        * @return void
+        */
+       private function apply_attributes_updates() {
+               if ( ! count( $this->lexical_updates ) ) {
+                       return;
+               }
+
+               /*
+                * Attribute updates can be enqueued in any order but updates
+                * to the document must occur in lexical order; that is, each
+                * replacement must be made before all others which follow it
+                * at later string indices in the input document.
+                *
+                * Sorting avoid making out-of-order replacements which
+                * can lead to mangled output, partially-duplicated
+                * attributes, and overwritten attributes.
+                */
+               usort( $this->lexical_updates, array( self::class, 'sort_start_ascending' ) );
+
+               foreach ( $this->lexical_updates as $diff ) {
+                       $this->output_buffer       .= substr( $this->html, $this->bytes_already_copied, $diff->start - $this->bytes_already_copied );
+                       $this->output_buffer       .= $diff->text;
+                       $this->bytes_already_copied = $diff->end;
+               }
+
+               /*
+                * Adjust bookmark locations to account for how the text
+                * replacements adjust offsets in the input document.
+                */
+               foreach ( $this->bookmarks as $bookmark ) {
+                       /*
+                        * Each lexical update which appears before the bookmark's endpoints
+                        * might shift the offsets for those endpoints. Loop through each change
+                        * and accumulate the total shift for each bookmark, then apply that
+                        * shift after tallying the full delta.
+                        */
+                       $head_delta = 0;
+                       $tail_delta = 0;
+
+                       foreach ( $this->lexical_updates as $diff ) {
+                               $update_head = $bookmark->start >= $diff->start;
+                               $update_tail = $bookmark->end >= $diff->start;
+
+                               if ( ! $update_head && ! $update_tail ) {
+                                       break;
+                               }
+
+                               $delta = strlen( $diff->text ) - ( $diff->end - $diff->start );
+
+                               if ( $update_head ) {
+                                       $head_delta += $delta;
+                               }
+
+                               if ( $update_tail ) {
+                                       $tail_delta += $delta;
+                               }
+                       }
+
+                       $bookmark->start += $head_delta;
+                       $bookmark->end   += $tail_delta;
+               }
+
+               $this->lexical_updates = array();
+       }
+
+       /**
+        * Move the internal cursor in the Tag Processor to a given bookmark's location.
+        *
+        * In order to prevent accidental infinite loops, there's a
+        * maximum limit on the number of times seek() can be called.
+        *
+        * @since 6.2.0
+        *
+        * @param string $bookmark_name Jump to the place in the document identified by this bookmark name.
+        * @return bool Whether the internal cursor was successfully moved to the bookmark's location.
+        */
+       public function seek( $bookmark_name ) {
+               if ( ! array_key_exists( $bookmark_name, $this->bookmarks ) ) {
+                       _doing_it_wrong(
+                               __METHOD__,
+                               __( 'Unknown bookmark name.' ),
+                               '6.2.0'
+                       );
+                       return false;
+               }
+
+               if ( ++$this->seek_count > self::MAX_SEEK_OPS ) {
+                       _doing_it_wrong(
+                               __METHOD__,
+                               __( 'Too many calls to seek() - this can lead to performance issues.' ),
+                               '6.2.0'
+                       );
+                       return false;
+               }
+
+               // Flush out any pending updates to the document.
+               $this->get_updated_html();
+
+               // Point this tag processor before the sought tag opener and consume it.
+               $this->bytes_already_parsed = $this->bookmarks[ $bookmark_name ]->start;
+               $this->bytes_already_copied = $this->bytes_already_parsed;
+               $this->output_buffer        = substr( $this->html, 0, $this->bytes_already_copied );
+               return $this->next_tag();
+       }
+
+       /**
+        * Compare two WP_HTML_Text_Replacement objects.
+        *
+        * @since 6.2.0
+        *
+        * @param WP_HTML_Text_Replacement $a First attribute update.
+        * @param WP_HTML_Text_Replacement $b Second attribute update.
+        * @return int Comparison value for string order.
+        */
+       private static function sort_start_ascending( $a, $b ) {
+               $by_start = $a->start - $b->start;
+               if ( 0 !== $by_start ) {
+                       return $by_start;
+               }
+
+               $by_text = isset( $a->text, $b->text ) ? strcmp( $a->text, $b->text ) : 0;
+               if ( 0 !== $by_text ) {
+                       return $by_text;
+               }
+
+               /*
+                * This code should be unreachable, because it implies the two replacements
+                * start at the same location and contain the same text.
+                */
+               return $a->end - $b->end;
+       }
+
+       /**
+        * Return the enqueued value for a given attribute, if one exists.
+        *
+        * Enqueued updates can take different data types:
+        *  - If an update is enqueued and is boolean, the return will be `true`
+        *  - If an update is otherwise enqueued, the return will be the string value of that update.
+        *  - If an attribute is enqueued to be removed, the return will be `null` to indicate that.
+        *  - If no updates are enqueued, the return will be `false` to differentiate from "removed."
+        *
+        * @since 6.2.0
+        *
+        * @param string $comparable_name The attribute name in its comparable form.
+        * @return string|boolean|null Value of enqueued update if present, otherwise false.
+        */
+       private function get_enqueued_attribute_value( $comparable_name ) {
+               if ( ! isset( $this->lexical_updates[ $comparable_name ] ) ) {
+                       return false;
+               }
+
+               $enqueued_text = $this->lexical_updates[ $comparable_name ]->text;
+
+               // Removed attributes erase the entire span.
+               if ( '' === $enqueued_text ) {
+                       return null;
+               }
+
+               /*
+                * Boolean attribute updates are just the attribute name without a corresponding value.
+                *
+                * This value might differ from the given comparable name in that there could be leading
+                * or trailing whitespace, and that the casing follows the name given in `set_attribute`.
+                *
+                * Example:
+                * ```
+                *     $p->set_attribute( 'data-TEST-id', 'update' );
+                *     'update' === $p->get_enqueued_attribute_value( 'data-test-id' );
+                * ```
+                *
+                * Detect this difference based on the absence of the `=`, which _must_ exist in any
+                * attribute containing a value, e.g. `<input type="text" enabled />`.
+                *                                            ¹           ²
+                *                                       1. Attribute with a string value.
+                *                                       2. Boolean attribute whose value is `true`.
+                */
+               $equals_at = strpos( $enqueued_text, '=' );
+               if ( false === $equals_at ) {
+                       return true;
+               }
+
+               /*
+                * Finally, a normal update's value will appear after the `=` and
+                * be double-quoted, as performed incidentally by `set_attribute`.
+                *
+                * e.g. `type="text"`
+                *           ¹²    ³
+                *        1. Equals is here.
+                *        2. Double-quoting starts one after the equals sign.
+                *        3. Double-quoting ends at the last character in the update.
+                */
+               $enqueued_value = substr( $enqueued_text, $equals_at + 2, -1 );
+               return html_entity_decode( $enqueued_value );
+       }
+
+       /**
+        * Returns the value of a requested attribute from a matched tag opener if that attribute exists.
+        *
+        * Example:
+        * ```php
+        *     $p = new WP_HTML_Tag_Processor( '<div enabled class="test" data-test-id="14">Test</div>' );
+        *     $p->next_tag( [ 'class_name' => 'test' ] ) === true;
+        *     $p->get_attribute( 'data-test-id' ) === '14';
+        *     $p->get_attribute( 'enabled' ) === true;
+        *     $p->get_attribute( 'aria-label' ) === null;
+        *
+        *     $p->next_tag( [] ) === false;
+        *     $p->get_attribute( 'class' ) === null;
+        * ```
+        *
+        * @since 6.2.0
+        *
+        * @param string $name Name of attribute whose value is requested.
+        * @return string|true|null Value of attribute or `null` if not available. Boolean attributes return `true`.
+        */
+       public function get_attribute( $name ) {
+               if ( null === $this->tag_name_starts_at ) {
+                       return null;
+               }
+
+               $comparable = strtolower( $name );
+
+               /*
+                * For every attribute other than `class` it's possible to perform a quick check if
+                * there's an enqueued lexical update whose value takes priority over what's found in
+                * the input document.
+                *
+                * The `class` attribute is special though because of the exposed helpers `add_class`
+                * and `remove_class`. These form a builder for the `class` attribute, so an additional
+                * check for enqueued class changes is required in addition to the check for any enqueued
+                * attribute values. If any exist, those enqueued class changes must first be flushed out
+                * into an attribute value update.
+                */
+               if ( 'class' === $name ) {
+                       $this->class_name_updates_to_attributes_updates();
+               }
+
+               // Return any enqueued attribute value updates if they exist.
+               $enqueued_value = $this->get_enqueued_attribute_value( $comparable );
+               if ( false !== $enqueued_value ) {
+                       return $enqueued_value;
+               }
+
+               if ( ! isset( $this->attributes[ $comparable ] ) ) {
+                       return null;
+               }
+
+               $attribute = $this->attributes[ $comparable ];
+
+               /*
+                * This flag distinguishes an attribute with no value
+                * from an attribute with an empty string value. For
+                * unquoted attributes this could look very similar.
+                * It refers to whether an `=` follows the name.
+                *
+                * e.g. <div boolean-attribute empty-attribute=></div>
+                *           ¹                 ²
+                *        1. Attribute `boolean-attribute` is `true`.
+                *        2. Attribute `empty-attribute` is `""`.
+                */
+               if ( true === $attribute->is_true ) {
+                       return true;
+               }
+
+               $raw_value = substr( $this->html, $attribute->value_starts_at, $attribute->value_length );
+
+               return html_entity_decode( $raw_value );
+       }
+
+       /**
+        * Gets lowercase names of all attributes matching a given prefix in the current tag.
+        *
+        * Note that matching is case-insensitive. This is in accordance with the spec:
+        *
+        * > There must never be two or more attributes on
+        * > the same start tag whose names are an ASCII
+        * > case-insensitive match for each other.
+        *     - HTML 5 spec
+        *
+        * @see https://html.spec.whatwg.org/multipage/syntax.html#attributes-2:ascii-case-insensitive
+        *
+        * Example:
+        * ```php
+        *     $p = new WP_HTML_Tag_Processor( '<div data-ENABLED class="test" DATA-test-id="14">Test</div>' );
+        *     $p->next_tag( [ 'class_name' => 'test' ] ) === true;
+        *     $p->get_attribute_names_with_prefix( 'data-' ) === array( 'data-enabled', 'data-test-id' );
+        *
+        *     $p->next_tag( [] ) === false;
+        *     $p->get_attribute_names_with_prefix( 'data-' ) === null;
+        * ```
+        *
+        * @since 6.2.0
+        *
+        * @param string $prefix Prefix of requested attribute names.
+        * @return array|null List of attribute names, or `null` when no tag opener is matched.
+        */
+       function get_attribute_names_with_prefix( $prefix ) {
+               if ( $this->is_closing_tag || null === $this->tag_name_starts_at ) {
+                       return null;
+               }
+
+               $comparable = strtolower( $prefix );
+
+               $matches = array();
+               foreach ( array_keys( $this->attributes ) as $attr_name ) {
+                       if ( str_starts_with( $attr_name, $comparable ) ) {
+                               $matches[] = $attr_name;
+                       }
+               }
+               return $matches;
+       }
+
+       /**
+        * Returns the uppercase name of the matched tag.
+        *
+        * Example:
+        * ```php
+        *     $p = new WP_HTML_Tag_Processor( '<DIV CLASS="test">Test</DIV>' );
+        *     $p->next_tag( [] ) === true;
+        *     $p->get_tag() === 'DIV';
+        *
+        *     $p->next_tag( [] ) === false;
+        *     $p->get_tag() === null;
+        * ```
+        *
+        * @since 6.2.0
+        *
+        * @return string|null Name of currently matched tag in input HTML, or `null` if none found.
+        */
+       public function get_tag() {
+               if ( null === $this->tag_name_starts_at ) {
+                       return null;
+               }
+
+               $tag_name = substr( $this->html, $this->tag_name_starts_at, $this->tag_name_length );
+
+               return strtoupper( $tag_name );
+       }
+
+       /**
+        * Indicates if the current tag token is a tag closer.
+        *
+        * Example:
+        * ```php
+        *     $p = new WP_HTML_Tag_Processor( '<div></div>' );
+        *     $p->next_tag( [ 'tag_name' => 'div', 'tag_closers' => 'visit' ] );
+        *     $p->is_tag_closer() === false;
+        *
+        *     $p->next_tag( [ 'tag_name' => 'div', 'tag_closers' => 'visit' ] );
+        *     $p->is_tag_closer() === true;
+        * ```
+        *
+        * @since 6.2.0
+        *
+        * @return bool Whether the current tag is a tag closer.
+        */
+       public function is_tag_closer() {
+               return $this->is_closing_tag;
+       }
+
+       /**
+        * Updates or creates a new attribute on the currently matched tag with the passed value.
+        *
+        * For boolean attributes special handling is provided:
+        *  - When `true` is passed as the value, then only the attribute name is added to the tag.
+        *  - When `false` is passed, the attribute gets removed if it existed before.
+        *
+        * For string attributes, the value is escaped using the `esc_attr` function.
+        *
+        * @since 6.2.0
+        *
+        * @param string      $name  The attribute name to target.
+        * @param string|bool $value The new attribute value.
+        * @return bool Whether an attribute value was set.
+        */
+       public function set_attribute( $name, $value ) {
+               if ( $this->is_closing_tag || null === $this->tag_name_starts_at ) {
+                       return false;
+               }
+
+               /*
+                * WordPress rejects more characters than are strictly forbidden
+                * in HTML5. This is to prevent additional security risks deeper
+                * in the WordPress and plugin stack. Specifically the
+                * less-than (<) greater-than (>) and ampersand (&) aren't allowed.
+                *
+                * The use of a PCRE match enables looking for specific Unicode
+                * code points without writing a UTF-8 decoder. Whereas scanning
+                * for one-byte characters is trivial (with `strcspn`), scanning
+                * for the longer byte sequences would be more complicated. Given
+                * that this shouldn't be in the hot path for execution, it's a
+                * reasonable compromise in efficiency without introducing a
+                * noticeable impact on the overall system.
+                *
+                * @see https://html.spec.whatwg.org/#attributes-2
+                *
+                * @TODO as the only regex pattern maybe we should take it out? are
+                *       Unicode patterns available broadly in Core?
+                */
+               if ( preg_match(
+                       '~[' .
+                               // Syntax-like characters.
+                               '"\'>&</ =' .
+                               // Control characters.
+                               '\x{00}-\x{1F}' .
+                               // HTML noncharacters.
+                               '\x{FDD0}-\x{FDEF}' .
+                               '\x{FFFE}\x{FFFF}\x{1FFFE}\x{1FFFF}\x{2FFFE}\x{2FFFF}\x{3FFFE}\x{3FFFF}' .
+                               '\x{4FFFE}\x{4FFFF}\x{5FFFE}\x{5FFFF}\x{6FFFE}\x{6FFFF}\x{7FFFE}\x{7FFFF}' .
+                               '\x{8FFFE}\x{8FFFF}\x{9FFFE}\x{9FFFF}\x{AFFFE}\x{AFFFF}\x{BFFFE}\x{BFFFF}' .
+                               '\x{CFFFE}\x{CFFFF}\x{DFFFE}\x{DFFFF}\x{EFFFE}\x{EFFFF}\x{FFFFE}\x{FFFFF}' .
+                               '\x{10FFFE}\x{10FFFF}' .
+                       ']~Ssu',
+                       $name
+               ) ) {
+                       _doing_it_wrong(
+                               __METHOD__,
+                               __( 'Invalid attribute name.' ),
+                               '6.2.0'
+                       );
+
+                       return false;
+               }
+
+               /*
+                * > The values "true" and "false" are not allowed on boolean attributes.
+                * > To represent a false value, the attribute has to be omitted altogether.
+                *     - HTML5 spec, https://html.spec.whatwg.org/#boolean-attributes
+                */
+               if ( false === $value ) {
+                       return $this->remove_attribute( $name );
+               }
+
+               if ( true === $value ) {
+                       $updated_attribute = $name;
+               } else {
+                       $escaped_new_value = esc_attr( $value );
+                       $updated_attribute = "{$name}=\"{$escaped_new_value}\"";
+               }
+
+               /*
+                * > There must never be two or more attributes on
+                * > the same start tag whose names are an ASCII
+                * > case-insensitive match for each other.
+                *     - HTML 5 spec
+                *
+                * @see https://html.spec.whatwg.org/multipage/syntax.html#attributes-2:ascii-case-insensitive
+                */
+               $comparable_name = strtolower( $name );
+
+               if ( isset( $this->attributes[ $comparable_name ] ) ) {
+                       /*
+                        * Update an existing attribute.
+                        *
+                        * Example – set attribute id to "new" in <div id="initial_id" />:
+                        *    <div id="initial_id"/>
+                        *         ^-------------^
+                        *         start         end
+                        *    replacement: `id="new"`
+                        *
+                        *    Result: <div id="new"/>
+                        */
+                       $existing_attribute             = $this->attributes[ $comparable_name ];
+                       $this->lexical_updates[ $name ] = new WP_HTML_Text_Replacement(
+                               $existing_attribute->start,
+                               $existing_attribute->end,
+                               $updated_attribute
+                       );
+               } else {
+                       /*
+                        * Create a new attribute at the tag's name end.
+                        *
+                        * Example – add attribute id="new" to <div />:
+                        *    <div/>
+                        *        ^
+                        *        start and end
+                        *    replacement: ` id="new"`
+                        *
+                        *    Result: <div id="new"/>
+                        */
+                       $this->lexical_updates[ $comparable_name ] = new WP_HTML_Text_Replacement(
+                               $this->tag_name_starts_at + $this->tag_name_length,
+                               $this->tag_name_starts_at + $this->tag_name_length,
+                               ' ' . $updated_attribute
+                       );
+               }
+
+               /*
+                * Any calls to update the `class` attribute directly should wipe out any
+                * enqueued class changes from `add_class` and `remove_class`.
+                */
+               if ( 'class' === $comparable_name && ! empty( $this->classname_updates ) ) {
+                       $this->classname_updates = array();
+               }
+
+               return true;
+       }
+
+       /**
+        * Remove an attribute from the currently-matched tag.
+        *
+        * @since 6.2.0
+        *
+        * @param string $name The attribute name to remove.
+        * @return bool Whether an attribute was removed.
+        */
+       public function remove_attribute( $name ) {
+               if ( $this->is_closing_tag ) {
+                       return false;
+               }
+
+               /*
+                * > There must never be two or more attributes on
+                * > the same start tag whose names are an ASCII
+                * > case-insensitive match for each other.
+                *     - HTML 5 spec
+                *
+                * @see https://html.spec.whatwg.org/multipage/syntax.html#attributes-2:ascii-case-insensitive
+                */
+               $name = strtolower( $name );
+
+               /*
+                * Any calls to update the `class` attribute directly should wipe out any
+                * enqueued class changes from `add_class` and `remove_class`.
+                */
+               if ( 'class' === $name && count( $this->classname_updates ) !== 0 ) {
+                       $this->classname_updates = array();
+               }
+
+               /*
+                * If updating an attribute that didn't exist in the input
+                * document, then remove the enqueued update and move on.
+                *
+                * For example, this might occur when calling `remove_attribute()`
+                * after calling `set_attribute()` for the same attribute
+                * and when that attribute wasn't originally present.
+                */
+               if ( ! isset( $this->attributes[ $name ] ) ) {
+                       if ( isset( $this->lexical_updates[ $name ] ) ) {
+                               unset( $this->lexical_updates[ $name ] );
+                       }
+                       return false;
+               }
+
+               /*
+                * Removes an existing tag attribute.
+                *
+                * Example – remove the attribute id from <div id="main"/>:
+                *    <div id="initial_id"/>
+                *         ^-------------^
+                *         start         end
+                *    replacement: ``
+                *
+                *    Result: <div />
+                */
+               $this->lexical_updates[ $name ] = new WP_HTML_Text_Replacement(
+                       $this->attributes[ $name ]->start,
+                       $this->attributes[ $name ]->end,
+                       ''
+               );
+
+               return true;
+       }
+
+       /**
+        * Adds a new class name to the currently matched tag.
+        *
+        * @since 6.2.0
+        *
+        * @param string $class_name The class name to add.
+        * @return bool Whether the class was set to be added.
+        */
+       public function add_class( $class_name ) {
+               if ( $this->is_closing_tag ) {
+                       return false;
+               }
+
+               if ( null !== $this->tag_name_starts_at ) {
+                       $this->classname_updates[ $class_name ] = self::ADD_CLASS;
+               }
+
+               return true;
+       }
+
+       /**
+        * Removes a class name from the currently matched tag.
+        *
+        * @since 6.2.0
+        *
+        * @param string $class_name The class name to remove.
+        * @return bool Whether the class was set to be removed.
+        */
+       public function remove_class( $class_name ) {
+               if ( $this->is_closing_tag ) {
+                       return false;
+               }
+
+               if ( null !== $this->tag_name_starts_at ) {
+                       $this->classname_updates[ $class_name ] = self::REMOVE_CLASS;
+               }
+
+               return true;
+       }
+
+       /**
+        * Returns the string representation of the HTML Tag Processor.
+        *
+        * @since 6.2.0
+        * @see get_updated_html
+        *
+        * @return string The processed HTML.
+        */
+       public function __toString() {
+               return $this->get_updated_html();
+       }
+
+       /**
+        * Returns the string representation of the HTML Tag Processor.
+        *
+        * @since 6.2.0
+        *
+        * @return string The processed HTML.
+        */
+       public function get_updated_html() {
+               $requires_no_updating = 0 === count( $this->classname_updates ) && 0 === count( $this->lexical_updates );
+
+               /*
+                * When there is nothing more to update and nothing has already been
+                * updated, return the original document and avoid a string copy.
+                */
+               if ( $requires_no_updating && 0 === $this->bytes_already_copied ) {
+                       return $this->html;
+               }
+
+               /*
+                * If there are no updates left to apply, but some have already
+                * been applied, then finish by copying the rest of the input
+                * to the end of the updated document and return.
+                */
+               if ( $requires_no_updating && $this->bytes_already_copied > 0 ) {
+                       return $this->output_buffer . substr( $this->html, $this->bytes_already_copied );
+               }
+
+               // Apply the updates, rewind to before the current tag, and reparse the attributes.
+               $content_up_to_opened_tag_name = $this->output_buffer . substr(
+                       $this->html,
+                       $this->bytes_already_copied,
+                       $this->tag_name_starts_at + $this->tag_name_length - $this->bytes_already_copied
+               );
+
+               /*
+                * 1. Apply the edits by flushing them to the output buffer and updating the copied byte count.
+                *
+                * Note: `apply_attributes_updates()` modifies `$this->output_buffer`.
+                */
+               $this->class_name_updates_to_attributes_updates();
+               $this->apply_attributes_updates();
+
+               /*
+                * 2. Replace the original HTML with the now-updated HTML so that it's possible to
+                *    seek to a previous location and have a consistent view of the updated document.
+                */
+               $this->html                 = $this->output_buffer . substr( $this->html, $this->bytes_already_copied );
+               $this->output_buffer        = $content_up_to_opened_tag_name;
+               $this->bytes_already_copied = strlen( $this->output_buffer );
+
+               /*
+                * 3. Point this tag processor at the original tag opener and consume it
+                *
+                * At this point the internal cursor points to the end of the tag name.
+                * Rewind before the tag name starts so that it's as if the cursor didn't
+                * move; a call to `next_tag()` will reparse the recently-updated attributes
+                * and additional calls to modify the attributes will apply at this same
+                * location.
+                *
+                * <p>Previous HTML<em>More HTML</em></p>
+                *                 ^  | back up by the length of the tag name plus the opening <
+                *                 \<-/ back up by strlen("em") + 1 ==> 3
+                */
+               $this->bytes_already_parsed = strlen( $content_up_to_opened_tag_name ) - $this->tag_name_length - 1;
+               $this->next_tag();
+
+               return $this->html;
+       }
+
+       /**
+        * Parses tag query input into internal search criteria.
+        *
+        * @since 6.2.0
+        *
+        * @param array|string|null $query {
+        *     Optional. Which tag name to find, having which class, etc. Default is to find any tag.
+        *
+        *     @type string|null $tag_name     Which tag to find, or `null` for "any tag."
+        *     @type int|null    $match_offset Find the Nth tag matching all search criteria.
+        *                                     0 for "first" tag, 2 for "third," etc.
+        *                                     Defaults to first tag.
+        *     @type string|null $class_name   Tag must contain this class name to match.
+        *     @type string      $tag_closers  "visit" or "skip": whether to stop on tag closers, e.g. </div>.
+        * }
+        * @return void
+        */
+       private function parse_query( $query ) {
+               if ( null !== $query && $query === $this->last_query ) {
+                       return;
+               }
+
+               $this->last_query          = $query;
+               $this->sought_tag_name     = null;
+               $this->sought_class_name   = null;
+               $this->sought_match_offset = 1;
+               $this->stop_on_tag_closers = false;
+
+               // A single string value means "find the tag of this name".
+               if ( is_string( $query ) ) {
+                       $this->sought_tag_name = $query;
+                       return;
+               }
+
+               // An empty query parameter applies no restrictions on the search.
+               if ( null === $query ) {
+                       return;
+               }
+
+               // If not using the string interface, an associative array is required.
+               if ( ! is_array( $query ) ) {
+                       _doing_it_wrong(
+                               __METHOD__,
+                               __( 'The query argument must be an array or a tag name.' ),
+                               '6.2.0'
+                       );
+                       return;
+               }
+
+               if ( isset( $query['tag_name'] ) && is_string( $query['tag_name'] ) ) {
+                       $this->sought_tag_name = $query['tag_name'];
+               }
+
+               if ( isset( $query['class_name'] ) && is_string( $query['class_name'] ) ) {
+                       $this->sought_class_name = $query['class_name'];
+               }
+
+               if ( isset( $query['match_offset'] ) && is_int( $query['match_offset'] ) && 0 < $query['match_offset'] ) {
+                       $this->sought_match_offset = $query['match_offset'];
+               }
+
+               if ( isset( $query['tag_closers'] ) ) {
+                       $this->stop_on_tag_closers = 'visit' === $query['tag_closers'];
+               }
+       }
+
+
+       /**
+        * Checks whether a given tag and its attributes match the search criteria.
+        *
+        * @since 6.2.0
+        *
+        * @return boolean Whether the given tag and its attribute match the search criteria.
+        */
+       private function matches() {
+               if ( $this->is_closing_tag && ! $this->stop_on_tag_closers ) {
+                       return false;
+               }
+
+               // Does the tag name match the requested tag name in a case-insensitive manner?
+               if ( null !== $this->sought_tag_name ) {
+                       /*
+                        * String (byte) length lookup is fast. If they aren't the
+                        * same length then they can't be the same string values.
+                        */
+                       if ( strlen( $this->sought_tag_name ) !== $this->tag_name_length ) {
+                               return false;
+                       }
+
+                       /*
+                        * Check each character to determine if they are the same.
+                        * Defer calls to `strtoupper()` to avoid them when possible.
+                        * Calling `strcasecmp()` here tested slowed than comparing each
+                        * character, so unless benchmarks show otherwise, it should
+                        * not be used.
+                        *
+                        * It's expected that most of the time that this runs, a
+                        * lower-case tag name will be supplied and the input will
+                        * contain lower-case tag names, thus normally bypassing
+                        * the case comparison code.
+                        */
+                       for ( $i = 0; $i < $this->tag_name_length; $i++ ) {
+                               $html_char = $this->html[ $this->tag_name_starts_at + $i ];
+                               $tag_char  = $this->sought_tag_name[ $i ];
+
+                               if ( $html_char !== $tag_char && strtoupper( $html_char ) !== $tag_char ) {
+                                       return false;
+                               }
+                       }
+               }
+
+               $needs_class_name = null !== $this->sought_class_name;
+
+               if ( $needs_class_name && ! isset( $this->attributes['class'] ) ) {
+                       return false;
+               }
+
+               /*
+                * Match byte-for-byte (case-sensitive and encoding-form-sensitive) on the class name.
+                *
+                * This will overlook certain classes that exist in other lexical variations
+                * than was supplied to the search query, but requires more complicated searching.
+                */
+               if ( $needs_class_name ) {
+                       $class_start = $this->attributes['class']->value_starts_at;
+                       $class_end   = $class_start + $this->attributes['class']->value_length;
+                       $class_at    = $class_start;
+
+                       /*
+                        * Ensure that boundaries surround the class name to avoid matching on
+                        * substrings of a longer name. For example, the sequence "not-odd"
+                        * should not match for the class "odd" even though "odd" is found
+                        * within the class attribute text.
+                        *
+                        * See https://html.spec.whatwg.org/#attributes-3
+                        * See https://html.spec.whatwg.org/#space-separated-tokens
+                        */
+                       while (
+                               // phpcs:ignore WordPress.CodeAnalysis.AssignmentInCondition.FoundInWhileCondition
+                               false !== ( $class_at = strpos( $this->html, $this->sought_class_name, $class_at ) ) &&
+                               $class_at < $class_end
+                       ) {
+                               /*
+                                * Verify this class starts at a boundary.
+                                */
+                               if ( $class_at > $class_start ) {
+                                       $character = $this->html[ $class_at - 1 ];
+
+                                       if ( ' ' !== $character && "\t" !== $character && "\f" !== $character && "\r" !== $character && "\n" !== $character ) {
+                                               $class_at += strlen( $this->sought_class_name );
+                                               continue;
+                                       }
+                               }
+
+                               /*
+                                * Verify this class ends at a boundary as well.
+                                */
+                               if ( $class_at + strlen( $this->sought_class_name ) < $class_end ) {
+                                       $character = $this->html[ $class_at + strlen( $this->sought_class_name ) ];
+
+                                       if ( ' ' !== $character && "\t" !== $character && "\f" !== $character && "\r" !== $character && "\n" !== $character ) {
+                                               $class_at += strlen( $this->sought_class_name );
+                                               continue;
+                                       }
+                               }
+
+                               return true;
+                       }
+
+                       return false;
+               }
+
+               return true;
+       }
+}
</ins></span></pre></div>
<a id="trunksrcwpincludeshtmlapiclasswphtmltextreplacementphp"></a>
<div class="addfile"><h4 style="background-color: #eee; color: inherit; margin: 1em 0; padding: 1.3em; font-size: 115%">Added: trunk/src/wp-includes/html-api/class-wp-html-text-replacement.php</h4>
<pre class="diff"><span>
<span class="info" style="display: block; padding: 0 10px; color: #888">--- trunk/src/wp-includes/html-api/class-wp-html-text-replacement.php                         (rev 0)
+++ trunk/src/wp-includes/html-api/class-wp-html-text-replacement.php   2023-02-03 01:03:59 UTC (rev 55203)
</span><span class="lines" style="display: block; padding: 0 10px; color: #888">@@ -0,0 +1,59 @@
</span><ins style="background-color: #dfd; text-decoration:none; display:block; padding: 0 10px">+<?php
+/**
+ * HTML Tag Processor: Text replacement class.
+ *
+ * @package WordPress
+ * @subpackage HTML-API
+ * @since 6.2.0
+ */
+
+/**
+ * Data structure used to replace existing content from start to end that allows to drastically improve performance.
+ *
+ * This class is for internal usage of the WP_HTML_Tag_Processor class.
+ *
+ * @access private
+ * @since 6.2.0
+ *
+ * @see WP_HTML_Tag_Processor
+ */
+class WP_HTML_Text_Replacement {
+       /**
+        * Byte offset into document where replacement span begins.
+        *
+        * @since 6.2.0
+        * @var int
+        */
+       public $start;
+
+       /**
+        * Byte offset into document where replacement span ends.
+        *
+        * @since 6.2.0
+        * @var int
+        */
+       public $end;
+
+       /**
+        * Span of text to insert in document to replace existing content from start to end.
+        *
+        * @since 6.2.0
+        * @var string
+        */
+       public $text;
+
+       /**
+        * Constructor.
+        *
+        * @since 6.2.0
+        *
+        * @param int    $start Byte offset into document where replacement span begins.
+        * @param int    $end   Byte offset into document where replacement span ends.
+        * @param string $text  Span of text to insert in document to replace existing content from start to end.
+        */
+       public function __construct( $start, $end, $text ) {
+               $this->start = $start;
+               $this->end   = $end;
+               $this->text  = $text;
+       }
+}
</ins></span></pre></div>
<a id="trunksrcwpsettingsphp"></a>
<div class="modfile"><h4 style="background-color: #eee; color: inherit; margin: 1em 0; padding: 1.3em; font-size: 115%">Modified: trunk/src/wp-settings.php</h4>
<pre class="diff"><span>
<span class="info" style="display: block; padding: 0 10px; color: #888">--- trunk/src/wp-settings.php 2023-02-03 00:45:24 UTC (rev 55202)
+++ trunk/src/wp-settings.php   2023-02-03 01:03:59 UTC (rev 55203)
</span><span class="lines" style="display: block; padding: 0 10px; color: #888">@@ -234,6 +234,10 @@
</span><span class="cx" style="display: block; padding: 0 10px"> require ABSPATH . WPINC . '/class-wp-oembed-controller.php';
</span><span class="cx" style="display: block; padding: 0 10px"> require ABSPATH . WPINC . '/media.php';
</span><span class="cx" style="display: block; padding: 0 10px"> require ABSPATH . WPINC . '/http.php';
</span><ins style="background-color: #dfd; text-decoration:none; display:block; padding: 0 10px">+require ABSPATH . WPINC . '/html-api/class-wp-html-attribute-token.php';
+require ABSPATH . WPINC . '/html-api/class-wp-html-span.php';
+require ABSPATH . WPINC . '/html-api/class-wp-html-text-replacement.php';
+require ABSPATH . WPINC . '/html-api/class-wp-html-tag-processor.php';
</ins><span class="cx" style="display: block; padding: 0 10px"> require ABSPATH . WPINC . '/class-wp-http.php';
</span><span class="cx" style="display: block; padding: 0 10px"> require ABSPATH . WPINC . '/class-wp-http-streams.php';
</span><span class="cx" style="display: block; padding: 0 10px"> require ABSPATH . WPINC . '/class-wp-http-curl.php';
</span></span></pre></div>
<a id="trunktestsphpunittestshtmlwpHtmlTagProcessorbookmarkphp"></a>
<div class="addfile"><h4 style="background-color: #eee; color: inherit; margin: 1em 0; padding: 1.3em; font-size: 115%">Added: trunk/tests/phpunit/tests/html/wpHtmlTagProcessor-bookmark.php</h4>
<pre class="diff"><span>
<span class="info" style="display: block; padding: 0 10px; color: #888">--- trunk/tests/phpunit/tests/html/wpHtmlTagProcessor-bookmark.php                            (rev 0)
+++ trunk/tests/phpunit/tests/html/wpHtmlTagProcessor-bookmark.php      2023-02-03 01:03:59 UTC (rev 55203)
</span><span class="lines" style="display: block; padding: 0 10px; color: #888">@@ -0,0 +1,381 @@
</span><ins style="background-color: #dfd; text-decoration:none; display:block; padding: 0 10px">+<?php
+/**
+ * Unit tests covering WP_HTML_Tag_Processor bookmark functionality.
+ *
+ * @package WordPress
+ * @subpackage HTML-API
+ */
+
+/**
+ * @group html-api
+ *
+ * @coversDefaultClass WP_HTML_Tag_Processor
+ */
+class Tests_HTML_wpHtmlTagProcessor_Bookmark extends WP_UnitTestCase {
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_bookmark
+        */
+       public function test_set_bookmark() {
+               $p = new WP_HTML_Tag_Processor( '<ul><li>One</li><li>Two</li><li>Three</li></ul>' );
+               $p->next_tag( 'li' );
+               $this->assertTrue( $p->set_bookmark( 'first li' ), 'Could not allocate a "first li" bookmark' );
+               $p->next_tag( 'li' );
+               $this->assertTrue( $p->set_bookmark( 'second li' ), 'Could not allocate a "second li" bookmark' );
+               $this->assertTrue( $p->set_bookmark( 'first li' ), 'Could not move the "first li" bookmark' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::release_bookmark
+        */
+       public function test_release_bookmark() {
+               $p = new WP_HTML_Tag_Processor( '<ul><li>One</li><li>Two</li><li>Three</li></ul>' );
+               $p->next_tag( 'li' );
+               $this->assertFalse( $p->release_bookmark( 'first li' ), 'Released a non-existing bookmark' );
+               $p->set_bookmark( 'first li' );
+               $this->assertTrue( $p->release_bookmark( 'first li' ), 'Could not release a bookmark' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::seek
+        */
+       public function test_seek() {
+               $p = new WP_HTML_Tag_Processor( '<ul><li>One</li><li>Two</li><li>Three</li></ul>' );
+               $p->next_tag( 'li' );
+               $p->set_bookmark( 'first li' );
+
+               $p->next_tag( 'li' );
+               $p->set_attribute( 'foo-2', 'bar-2' );
+
+               $p->seek( 'first li' );
+               $p->set_attribute( 'foo-1', 'bar-1' );
+
+               $this->assertSame(
+                       '<ul><li foo-1="bar-1">One</li><li foo-2="bar-2">Two</li><li>Three</li></ul>',
+                       $p->get_updated_html(),
+                       'Did not seek to the intended bookmark locations'
+               );
+       }
+
+       /**
+        * WP_HTML_Tag_Processor used to test for the diffs affecting
+        * the adjusted bookmark position while simultaneously adjusting
+        * the bookmark in question. As a result, updating the bookmarks
+        * of a next tag while removing two subsequent attributes in
+        * a previous tag unfolded like this:
+        *
+        * 1. Check if the first removed attribute is before the bookmark:
+        *
+        * <button twenty_one_characters 7_chars></button><button></button>
+        *         ^-------------------^                  ^
+        *           diff applied here           the bookmark is here
+        *
+        *    (Yes it is)
+        *
+        * 2. Move the bookmark to the left by the attribute length:
+        *
+        * <button twenty_one_characters 7_chars></button><button></button>
+        *                           ^
+        *                   the bookmark is here
+        *
+        * 3. Check if the second removed attribute is before the bookmark:
+        *
+        * <button twenty_one_characters 7_chars></button><button></button>
+        *                           ^   ^-----^
+        *                    bookmark    diff
+        *
+        *    This time, it isn't!
+        *
+        * The fix in the WP_HTML_Tag_Processor involves doing all the checks
+        * before moving the bookmark. This test is here to guard us from
+        * the erroneous behavior accidentally returning one day.
+        *
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::seek
+        * @covers WP_HTML_Tag_Processor::set_bookmark
+        */
+       public function test_removing_long_attributes_doesnt_break_seek() {
+               $input = <<<HTML
+               <button twenty_one_characters 7_chars></button><button></button>
+HTML;
+               $p     = new WP_HTML_Tag_Processor( $input );
+               $p->next_tag( 'button' );
+               $p->set_bookmark( 'first' );
+               $p->next_tag( 'button' );
+               $p->set_bookmark( 'second' );
+
+               $this->assertTrue(
+                       $p->seek( 'first' ),
+                       'Seek() to the first button has failed'
+               );
+               $p->remove_attribute( 'twenty_one_characters' );
+               $p->remove_attribute( '7_chars' );
+
+               $this->assertTrue(
+                       $p->seek( 'second' ),
+                       'Seek() to the second button has failed'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::seek
+        * @covers WP_HTML_Tag_Processor::set_bookmark
+        */
+       public function test_bookmarks_complex_use_case() {
+               $input           = <<<HTML
+<div selected class="merge-message" checked>
+       <div class="select-menu d-inline-block">
+               <div checked class="BtnGroup MixedCaseHTML position-relative" />
+               <div checked class="BtnGroup MixedCaseHTML position-relative">
+                       <button type="button" class="merge-box-button btn-group-merge rounded-left-2 btn  BtnGroup-item js-details-target hx_create-pr-button" aria-expanded="false" data-details-container=".js-merge-pr" disabled="">
+                         Merge pull request
+                       </button>
+
+                       <button type="button" class="merge-box-button btn-group-squash rounded-left-2 btn  BtnGroup-item js-details-target hx_create-pr-button" aria-expanded="false" data-details-container=".js-merge-pr" disabled="">
+                         Squash and merge
+                       </button>
+
+                       <button type="button" class="merge-box-button btn-group-rebase rounded-left-2 btn  BtnGroup-item js-details-target hx_create-pr-button" aria-expanded="false" data-details-container=".js-merge-pr" disabled="">
+                         Rebase and merge
+                       </button>
+
+                       <button aria-label="Select merge method" disabled="disabled" type="button" data-view-component="true" class="select-menu-button btn BtnGroup-item"></button>
+               </div>
+       </div>
+</div>
+HTML;
+               $expected_output = <<<HTML
+<div selected class="merge-message" checked>
+       <div class="select-menu d-inline-block">
+               <div  class="BtnGroup MixedCaseHTML position-relative" />
+               <div checked class="BtnGroup MixedCaseHTML position-relative">
+                       <button type="submit" class="merge-box-button btn-group-merge rounded-left-2 btn  BtnGroup-item js-details-target hx_create-pr-button" aria-expanded="false" data-details-container=".js-merge-pr" disabled="">
+                         Merge pull request
+                       </button>
+
+                       <button  class="hx_create-pr-button" aria-expanded="false" data-details-container=".js-merge-pr" disabled="">
+                         Squash and merge
+                       </button>
+
+                       <button id="rebase-and-merge"     disabled="">
+                         Rebase and merge
+                       </button>
+
+                       <button id="last-button"     ></button>
+               </div>
+       </div>
+</div>
+HTML;
+               $p               = new WP_HTML_Tag_Processor( $input );
+               $p->next_tag( 'div' );
+               $p->next_tag( 'div' );
+               $p->next_tag( 'div' );
+               $p->set_bookmark( 'first div' );
+               $p->next_tag( 'button' );
+               $p->set_bookmark( 'first button' );
+               $p->next_tag( 'button' );
+               $p->set_bookmark( 'second button' );
+               $p->next_tag( 'button' );
+               $p->set_bookmark( 'third button' );
+               $p->next_tag( 'button' );
+               $p->set_bookmark( 'fourth button' );
+
+               $p->seek( 'first button' );
+               $p->set_attribute( 'type', 'submit' );
+
+               $this->assertTrue(
+                       $p->seek( 'third button' ),
+                       'Seek() to the third button failed'
+               );
+               $p->remove_attribute( 'class' );
+               $p->remove_attribute( 'type' );
+               $p->remove_attribute( 'aria-expanded' );
+               $p->set_attribute( 'id', 'rebase-and-merge' );
+               $p->remove_attribute( 'data-details-container' );
+
+               $this->assertTrue(
+                       $p->seek( 'first div' ),
+                       'Seek() to the first div failed'
+               );
+               $p->set_attribute( 'checked', false );
+
+               $this->assertTrue(
+                       $p->seek( 'fourth button' ),
+                       'Seek() to the fourth button failed'
+               );
+               $p->set_attribute( 'id', 'last-button' );
+               $p->remove_attribute( 'class' );
+               $p->remove_attribute( 'type' );
+               $p->remove_attribute( 'checked' );
+               $p->remove_attribute( 'aria-label' );
+               $p->remove_attribute( 'disabled' );
+               $p->remove_attribute( 'data-view-component' );
+
+               $this->assertTrue(
+                       $p->seek( 'second button' ),
+                       'Seek() to the second button failed'
+               );
+               $p->remove_attribute( 'type' );
+               $p->set_attribute( 'class', 'hx_create-pr-button' );
+
+               $this->assertSame(
+                       $expected_output,
+                       $p->get_updated_html(),
+                       'Performing several attribute updates on different tags does not produce the expected HTML snippet'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::seek
+        */
+       public function test_updates_bookmark_for_additions_after_both_sides() {
+               $p = new WP_HTML_Tag_Processor( '<div>First</div><div>Second</div>' );
+               $p->next_tag();
+               $p->set_bookmark( 'first' );
+               $p->next_tag();
+               $p->add_class( 'second' );
+
+               $p->seek( 'first' );
+               $p->add_class( 'first' );
+
+               $this->assertSame(
+                       '<div class="first">First</div><div class="second">Second</div>',
+                       $p->get_updated_html(),
+                       'The bookmark was updated incorrectly in response to HTML markup updates'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::seek
+        */
+       public function test_updates_bookmark_for_additions_before_both_sides() {
+               $p = new WP_HTML_Tag_Processor( '<div>First</div><div>Second</div>' );
+               $p->next_tag();
+               $p->set_bookmark( 'first' );
+               $p->next_tag();
+               $p->set_bookmark( 'second' );
+
+               $p->seek( 'first' );
+               $p->add_class( 'first' );
+
+               $p->seek( 'second' );
+               $p->add_class( 'second' );
+
+               $this->assertSame(
+                       '<div class="first">First</div><div class="second">Second</div>',
+                       $p->get_updated_html(),
+                       'The bookmark was updated incorrectly in response to HTML markup updates'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::seek
+        */
+       public function test_updates_bookmark_for_deletions_after_both_sides() {
+               $p = new WP_HTML_Tag_Processor( '<div>First</div><div disabled>Second</div>' );
+               $p->next_tag();
+               $p->set_bookmark( 'first' );
+               $p->next_tag();
+               $p->remove_attribute( 'disabled' );
+
+               $p->seek( 'first' );
+               $p->set_attribute( 'untouched', true );
+
+               $this->assertSame(
+                       /*
+                        * It shouldn't be necessary to assert the extra space after the tag
+                        * following the attribute removal, but doing so makes the test easier
+                        * to see than it would be if parsing the output HTML for proper
+                        * validation. If the Tag Processor changes so that this space no longer
+                        * appears then this test should be updated to reflect that. The space
+                        * is not required.
+                        */
+                       '<div untouched>First</div><div >Second</div>',
+                       $p->get_updated_html(),
+                       'The bookmark was incorrectly in response to HTML markup updates'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::seek
+        */
+       public function test_updates_bookmark_for_deletions_before_both_sides() {
+               $p = new WP_HTML_Tag_Processor( '<div disabled>First</div><div>Second</div>' );
+               $p->next_tag();
+               $p->set_bookmark( 'first' );
+               $p->next_tag();
+               $p->set_bookmark( 'second' );
+
+               $p->seek( 'first' );
+               $p->remove_attribute( 'disabled' );
+
+               $p->seek( 'second' );
+               $p->set_attribute( 'safe', true );
+
+               $this->assertSame(
+                       /*
+                        * It shouldn't be necessary to assert the extra space after the tag
+                        * following the attribute removal, but doing so makes the test easier
+                        * to see than it would be if parsing the output HTML for proper
+                        * validation. If the Tag Processor changes so that this space no longer
+                        * appears then this test should be updated to reflect that. The space
+                        * is not required.
+                        */
+                       '<div >First</div><div safe>Second</div>',
+                       $p->get_updated_html(),
+                       'The bookmark was updated incorrectly in response to HTML markup updates'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_bookmark
+        */
+       public function test_limits_the_number_of_bookmarks() {
+               $p = new WP_HTML_Tag_Processor( '<ul><li>One</li><li>Two</li><li>Three</li></ul>' );
+               $p->next_tag( 'li' );
+
+               for ( $i = 0; $i < WP_HTML_Tag_Processor::MAX_BOOKMARKS; $i++ ) {
+                       $this->assertTrue( $p->set_bookmark( "bookmark $i" ), "Could not allocate the bookmark #$i" );
+               }
+
+               $this->setExpectedIncorrectUsage( 'WP_HTML_Tag_Processor::set_bookmark' );
+               $this->assertFalse( $p->set_bookmark( 'final bookmark' ), "Allocated $i bookmarks, which is one above the limit" );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::seek
+        */
+       public function test_limits_the_number_of_seek_calls() {
+               $p = new WP_HTML_Tag_Processor( '<ul><li>One</li><li>Two</li><li>Three</li></ul>' );
+               $p->next_tag( 'li' );
+               $p->set_bookmark( 'bookmark' );
+
+               for ( $i = 0; $i < WP_HTML_Tag_Processor::MAX_SEEK_OPS; $i++ ) {
+                       $this->assertTrue( $p->seek( 'bookmark' ), 'Could not seek to the "bookmark"' );
+               }
+
+               $this->setExpectedIncorrectUsage( 'WP_HTML_Tag_Processor::seek' );
+               $this->assertFalse( $p->seek( 'bookmark' ), "$i-th seek() to the bookmark succeeded, even though it should exceed the allowed limit" );
+       }
+}
</ins></span></pre></div>
<a id="trunktestsphpunittestshtmlwpHtmlTagProcessorphp"></a>
<div class="addfile"><h4 style="background-color: #eee; color: inherit; margin: 1em 0; padding: 1.3em; font-size: 115%">Added: trunk/tests/phpunit/tests/html/wpHtmlTagProcessor.php</h4>
<pre class="diff"><span>
<span class="info" style="display: block; padding: 0 10px; color: #888">--- trunk/tests/phpunit/tests/html/wpHtmlTagProcessor.php                             (rev 0)
+++ trunk/tests/phpunit/tests/html/wpHtmlTagProcessor.php       2023-02-03 01:03:59 UTC (rev 55203)
</span><span class="lines" style="display: block; padding: 0 10px; color: #888">@@ -0,0 +1,2016 @@
</span><ins style="background-color: #dfd; text-decoration:none; display:block; padding: 0 10px">+<?php
+/**
+ * Unit tests covering WP_HTML_Tag_Processor functionality.
+ *
+ * @package WordPress
+ * @subpackage HTML-API
+ */
+
+/**
+ * @group html-api
+ *
+ * @coversDefaultClass WP_HTML_Tag_Processor
+ */
+class Tests_HTML_wpHtmlTagProcessor extends WP_UnitTestCase {
+       const HTML_SIMPLE       = '<div id="first"><span id="second">Text</span></div>';
+       const HTML_WITH_CLASSES = '<div class="main with-border" id="first"><span class="not-main bold with-border" id="second">Text</span></div>';
+       const HTML_MALFORMED    = '<div><span class="d-md-none" Notifications</span><span class="d-none d-md-inline">Back to notifications</span></div>';
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_tag
+        */
+       public function test_get_tag_returns_null_before_finding_tags() {
+               $p = new WP_HTML_Tag_Processor( '<div>Test</div>' );
+
+               $this->assertNull( $p->get_tag(), 'Calling get_tag() without selecting a tag did not return null' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_tag
+        */
+       public function test_get_tag_returns_null_when_not_in_open_tag() {
+               $p = new WP_HTML_Tag_Processor( '<div>Test</div>' );
+
+               $this->assertFalse( $p->next_tag( 'p' ), 'Querying a non-existing tag did not return false' );
+               $this->assertNull( $p->get_tag(), 'Accessing a non-existing tag did not return null' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_tag
+        */
+       public function test_get_tag_returns_open_tag_name() {
+               $p = new WP_HTML_Tag_Processor( '<div>Test</div>' );
+
+               $this->assertTrue( $p->next_tag( 'div' ), 'Querying an existing tag did not return true' );
+               $this->assertSame( 'DIV', $p->get_tag(), 'Accessing an existing tag name did not return "div"' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_returns_null_before_finding_tags() {
+               $p = new WP_HTML_Tag_Processor( '<div class="test">Test</div>' );
+
+               $this->assertNull( $p->get_attribute( 'class' ), 'Accessing an attribute without selecting a tag did not return null' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_returns_null_when_not_in_open_tag() {
+               $p = new WP_HTML_Tag_Processor( '<div class="test">Test</div>' );
+
+               $this->assertFalse( $p->next_tag( 'p' ), 'Querying a non-existing tag did not return false' );
+               $this->assertNull( $p->get_attribute( 'class' ), 'Accessing an attribute of a non-existing tag did not return null' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_returns_null_when_in_closing_tag() {
+               $p = new WP_HTML_Tag_Processor( '<div class="test">Test</div>' );
+
+               $this->assertTrue( $p->next_tag( 'div' ), 'Querying an existing tag did not return true' );
+               $this->assertTrue( $p->next_tag( array( 'tag_closers' => 'visit' ) ), 'Querying an existing closing tag did not return true' );
+               $this->assertNull( $p->get_attribute( 'class' ), 'Accessing an attribute of a closing tag did not return null' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_returns_null_when_attribute_missing() {
+               $p = new WP_HTML_Tag_Processor( '<div class="test">Test</div>' );
+
+               $this->assertTrue( $p->next_tag( 'div' ), 'Querying an existing tag did not return true' );
+               $this->assertNull( $p->get_attribute( 'test-id' ), 'Accessing a non-existing attribute did not return null' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_returns_attribute_value() {
+               $p = new WP_HTML_Tag_Processor( '<div class="test">Test</div>' );
+
+               $this->assertTrue( $p->next_tag( 'div' ), 'Querying an existing tag did not return true' );
+               $this->assertSame( 'test', $p->get_attribute( 'class' ), 'Accessing a class="test" attribute value did not return "test"' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_returns_true_for_boolean_attribute() {
+               $p = new WP_HTML_Tag_Processor( '<div enabled class="test">Test</div>' );
+
+               $this->assertTrue( $p->next_tag( array( 'class_name' => 'test' ) ), 'Querying an existing tag did not return true' );
+               $this->assertTrue( $p->get_attribute( 'enabled' ), 'Accessing a boolean "enabled" attribute value did not return true' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_returns_string_for_truthy_attributes() {
+               $p = new WP_HTML_Tag_Processor( '<div enabled=enabled checked=1 hidden="true" class="test">Test</div>' );
+
+               $this->assertTrue( $p->next_tag(), 'Querying an existing tag did not return true' );
+               $this->assertSame( 'enabled', $p->get_attribute( 'enabled' ), 'Accessing a boolean "enabled" attribute value did not return true' );
+               $this->assertSame( '1', $p->get_attribute( 'checked' ), 'Accessing a checked=1 attribute value did not return "1"' );
+               $this->assertSame( 'true', $p->get_attribute( 'hidden' ), 'Accessing a hidden="true" attribute value did not return "true"' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_decodes_html_character_references() {
+               $p = new WP_HTML_Tag_Processor( '<div id="the &quot;grande&quot; is &lt; &#x033;&#50;oz&dagger;"></div>' );
+               $p->next_tag();
+
+               $this->assertSame( 'the "grande" is < 32oz†', $p->get_attribute( 'id' ), 'HTML Attribute value was returned without decoding character references' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_attributes_parser_treats_slash_as_attribute_separator() {
+               $p = new WP_HTML_Tag_Processor( '<div a/b/c/d/e="test">Test</div>' );
+
+               $this->assertTrue( $p->next_tag(), 'Querying an existing tag did not return true' );
+               $this->assertTrue( $p->get_attribute( 'a' ), 'Accessing an existing attribute did not return true' );
+               $this->assertTrue( $p->get_attribute( 'b' ), 'Accessing an existing attribute did not return true' );
+               $this->assertTrue( $p->get_attribute( 'c' ), 'Accessing an existing attribute did not return true' );
+               $this->assertTrue( $p->get_attribute( 'd' ), 'Accessing an existing attribute did not return true' );
+               $this->assertSame( 'test', $p->get_attribute( 'e' ), 'Accessing an existing e="test" did not return "test"' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        *
+        * @dataProvider data_attribute_name_case_variants
+        *
+        * @param string $attribute_name Name of data-enabled attribute with case variations.
+        */
+       public function test_get_attribute_is_case_insensitive_for_attributes_with_values( $attribute_name ) {
+               $p = new WP_HTML_Tag_Processor( '<div DATA-enabled="true">Test</div>' );
+               $p->next_tag();
+
+               $this->assertSame(
+                       'true',
+                       $p->get_attribute( $attribute_name ),
+                       'Accessing an attribute by a differently-cased name did not return its value'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        *
+        * @dataProvider data_attribute_name_case_variants
+        *
+        * @param string $attribute_name Name of data-enabled attribute with case variations.
+        */
+       public function test_attributes_parser_is_case_insensitive_for_attributes_without_values( $attribute_name ) {
+               $p = new WP_HTML_Tag_Processor( '<div DATA-enabled>Test</div>' );
+               $p->next_tag();
+
+               $this->assertTrue(
+                       $p->get_attribute( $attribute_name ),
+                       'Accessing an attribute by a differently-cased name did not return its value'
+               );
+       }
+
+       /**
+        * Data provider.
+        *
+        * @return array[].
+        */
+       public function data_attribute_name_case_variants() {
+               return array(
+                       array( 'DATA-enabled' ),
+                       array( 'data-enabled' ),
+                       array( 'DATA-ENABLED' ),
+                       array( 'DatA-EnABled' ),
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::remove_attribute
+        */
+       public function test_remove_attribute_is_case_insensitive() {
+               $p = new WP_HTML_Tag_Processor( '<div DATA-enabled="true">Test</div>' );
+               $p->next_tag();
+               $p->remove_attribute( 'data-enabled' );
+
+               $this->assertSame( '<div >Test</div>', $p->get_updated_html(), 'A case-insensitive remove_attribute call did not remove the attribute' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_set_attribute_is_case_insensitive() {
+               $p = new WP_HTML_Tag_Processor( '<div DATA-enabled="true">Test</div>' );
+               $p->next_tag();
+               $p->set_attribute( 'data-enabled', 'abc' );
+
+               $this->assertSame( '<div data-enabled="abc">Test</div>', $p->get_updated_html(), 'A case-insensitive set_attribute call did not update the existing attribute' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute_names_with_prefix
+        */
+       public function test_get_attribute_names_with_prefix_returns_null_before_finding_tags() {
+               $p = new WP_HTML_Tag_Processor( '<div data-foo="bar">Test</div>' );
+               $this->assertNull(
+                       $p->get_attribute_names_with_prefix( 'data-' ),
+                       'Accessing attributes by their prefix did not return null when no tag was selected'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute_names_with_prefix
+        */
+       public function test_get_attribute_names_with_prefix_returns_null_when_not_in_open_tag() {
+               $p = new WP_HTML_Tag_Processor( '<div data-foo="bar">Test</div>' );
+               $p->next_tag( 'p' );
+               $this->assertNull( $p->get_attribute_names_with_prefix( 'data-' ), 'Accessing attributes of a non-existing tag did not return null' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute_names_with_prefix
+        */
+       public function test_get_attribute_names_with_prefix_returns_null_when_in_closing_tag() {
+               $p = new WP_HTML_Tag_Processor( '<div data-foo="bar">Test</div>' );
+               $p->next_tag( 'div' );
+               $p->next_tag( array( 'tag_closers' => 'visit' ) );
+
+               $this->assertNull( $p->get_attribute_names_with_prefix( 'data-' ), 'Accessing attributes of a closing tag did not return null' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute_names_with_prefix
+        */
+       public function test_get_attribute_names_with_prefix_returns_empty_array_when_no_attributes_present() {
+               $p = new WP_HTML_Tag_Processor( '<div>Test</div>' );
+               $p->next_tag( 'div' );
+
+               $this->assertSame( array(), $p->get_attribute_names_with_prefix( 'data-' ), 'Accessing the attributes on a tag without any did not return an empty array' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute_names_with_prefix
+        */
+       public function test_get_attribute_names_with_prefix_returns_matching_attribute_names_in_lowercase() {
+               $p = new WP_HTML_Tag_Processor( '<div DATA-enabled class="test" data-test-ID="14">Test</div>' );
+               $p->next_tag();
+
+               $this->assertSame(
+                       array( 'data-enabled', 'data-test-id' ),
+                       $p->get_attribute_names_with_prefix( 'data-' ),
+                       'Accessing attributes by their prefix did not return their lowercase names'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute_names_with_prefix
+        */
+       public function test_get_attribute_names_with_prefix_returns_attribute_added_by_set_attribute() {
+               $p = new WP_HTML_Tag_Processor( '<div data-foo="bar">Test</div>' );
+               $p->next_tag();
+               $p->set_attribute( 'data-test-id', '14' );
+
+               $this->assertSame(
+                       '<div data-test-id="14" data-foo="bar">Test</div>',
+                       $p->get_updated_html(),
+                       "Updated HTML doesn't include attribute added via set_attribute"
+               );
+               $this->assertSame(
+                       array( 'data-test-id', 'data-foo' ),
+                       $p->get_attribute_names_with_prefix( 'data-' ),
+                       "Accessing attribute names doesn't find attribute added via set_attribute"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::__toString
+        */
+       public function test_to_string_returns_updated_html() {
+               $p = new WP_HTML_Tag_Processor( '<hr id="remove" /><div enabled class="test">Test</div><span id="span-id"></span>' );
+               $p->next_tag();
+               $p->remove_attribute( 'id' );
+
+               $p->next_tag();
+               $p->set_attribute( 'id', 'div-id-1' );
+               $p->add_class( 'new_class_1' );
+
+               $this->assertSame(
+                       $p->get_updated_html(),
+                       (string) $p,
+                       'get_updated_html() returned a different value than __toString()'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_updated_html
+        */
+       public function test_get_updated_html_applies_the_updates_so_far_and_keeps_the_processor_on_the_current_tag() {
+               $p = new WP_HTML_Tag_Processor( '<hr id="remove" /><div enabled class="test">Test</div><span id="span-id"></span>' );
+               $p->next_tag();
+               $p->remove_attribute( 'id' );
+
+               $p->next_tag();
+               $p->set_attribute( 'id', 'div-id-1' );
+               $p->add_class( 'new_class_1' );
+
+               $this->assertSame(
+                       '<hr  /><div id="div-id-1" enabled class="test new_class_1">Test</div><span id="span-id"></span>',
+                       $p->get_updated_html(),
+                       'Calling get_updated_html after updating the attributes of the second tag returned different HTML than expected'
+               );
+
+               $p->set_attribute( 'id', 'div-id-2' );
+               $p->add_class( 'new_class_2' );
+
+               $this->assertSame(
+                       '<hr  /><div id="div-id-2" enabled class="test new_class_1 new_class_2">Test</div><span id="span-id"></span>',
+                       $p->get_updated_html(),
+                       'Calling get_updated_html after updating the attributes of the second tag for the second time returned different HTML than expected'
+               );
+
+               $p->next_tag();
+               $p->remove_attribute( 'id' );
+
+               $this->assertSame(
+                       '<hr  /><div id="div-id-2" enabled class="test new_class_1 new_class_2">Test</div><span ></span>',
+                       $p->get_updated_html(),
+                       'Calling get_updated_html after removing the id attribute of the third tag returned different HTML than expected'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_updated_html
+        */
+       public function test_get_updated_html_without_updating_any_attributes_returns_the_original_html() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+
+               $this->assertSame(
+                       self::HTML_SIMPLE,
+                       $p->get_updated_html(),
+                       'Casting WP_HTML_Tag_Processor to a string without performing any updates did not return the initial HTML snippet'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::next_tag
+        */
+       public function test_next_tag_with_no_arguments_should_find_the_next_existing_tag() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+
+               $this->assertTrue( $p->next_tag(), 'Querying an existing tag did not return true' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::next_tag
+        */
+       public function test_next_tag_should_return_false_for_a_non_existing_tag() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+
+               $this->assertFalse( $p->next_tag( 'p' ), 'Querying a non-existing tag did not return false' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::next_tag
+        * @covers WP_HTML_Tag_Processor::is_tag_closer
+        */
+       public function test_next_tag_should_stop_on_closers_only_when_requested() {
+               $p = new WP_HTML_Tag_Processor( '<div><img /></div>' );
+
+               $this->assertTrue( $p->next_tag( array( 'tag_name' => 'div' ) ), 'Did not find desired tag opener' );
+               $this->assertFalse( $p->next_tag( array( 'tag_name' => 'div' ) ), 'Visited an unwanted tag, a tag closer' );
+
+               $p = new WP_HTML_Tag_Processor( '<div><img /></div>' );
+               $p->next_tag(
+                       array(
+                               'tag_name'    => 'div',
+                               'tag_closers' => 'visit',
+                       )
+               );
+
+               $this->assertFalse( $p->is_tag_closer(), 'Indicated a tag opener is a tag closer' );
+               $this->assertTrue(
+                       $p->next_tag(
+                               array(
+                                       'tag_name'    => 'div',
+                                       'tag_closers' => 'visit',
+                               )
+                       ),
+                       'Did not stop at desired tag closer'
+               );
+               $this->assertTrue( $p->is_tag_closer(), 'Indicated a tag closer is a tag opener' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_set_attribute_on_a_non_existing_tag_does_not_change_the_markup() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+
+               $this->assertFalse( $p->next_tag( 'p' ), 'Querying a non-existing tag did not return false' );
+               $this->assertFalse( $p->next_tag( 'div' ), 'Querying a non-existing tag did not return false' );
+
+               $p->set_attribute( 'id', 'primary' );
+
+               $this->assertSame(
+                       self::HTML_SIMPLE,
+                       $p->get_updated_html(),
+                       'Calling get_updated_html after updating a non-existing tag returned an HTML that was different from the original HTML'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        * @covers WP_HTML_Tag_Processor::remove_attribute
+        * @covers WP_HTML_Tag_Processor::add_class
+        * @covers WP_HTML_Tag_Processor::remove_class
+        */
+       public function test_attribute_ops_on_tag_closer_do_not_change_the_markup() {
+               $p = new WP_HTML_Tag_Processor( '<div id=3></div invalid-id=4>' );
+               $p->next_tag(
+                       array(
+                               'tag_name'    => 'div',
+                               'tag_closers' => 'visit',
+                       )
+               );
+
+               $this->assertFalse( $p->is_tag_closer(), 'Skipped tag opener' );
+
+               $p->next_tag(
+                       array(
+                               'tag_name'    => 'div',
+                               'tag_closers' => 'visit',
+                       )
+               );
+
+               $this->assertTrue( $p->is_tag_closer(), 'Skipped tag closer' );
+               $this->assertFalse( $p->set_attribute( 'id', 'test' ), "Allowed setting an attribute on a tag closer when it shouldn't have" );
+               $this->assertFalse( $p->remove_attribute( 'invalid-id' ), "Allowed removing an attribute on a tag closer when it shouldn't have" );
+               $this->assertFalse( $p->add_class( 'sneaky' ), "Allowed adding a class on a tag closer when it shouldn't have" );
+               $this->assertFalse( $p->remove_class( 'not-appearing-in-this-test' ), "Allowed removing a class on a tag closer when it shouldn't have" );
+               $this->assertSame(
+                       '<div id=3></div invalid-id=4>',
+                       $p->get_updated_html(),
+                       'Calling get_updated_html after updating a non-existing tag returned an HTML that was different from the original HTML'
+               );
+       }
+
+       /**
+        * Passing a double quote inside of an attribute value could lead to an XSS attack as follows:
+        *
+        * ```php
+        *     $p = new WP_HTML_Tag_Processor( '<div class="header"></div>' );
+        *     $p->next_tag();
+        *     $p->set_attribute('class', '" onclick="alert');
+        *     echo $p;
+        *     // <div class="" onclick="alert"></div>
+        * ```
+        *
+        * To prevent it, `set_attribute` calls `esc_attr()` on its given values.
+        *
+        * ```php
+        *    <div class="&quot; onclick=&quot;alert"></div>
+        * ```
+        *
+        * @ticket 56299
+        *
+        * @dataProvider data_set_attribute_prevents_xss
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        *
+        * @param string $attribute_value A value with potential XSS exploit.
+        */
+       public function test_set_attribute_prevents_xss( $attribute_value ) {
+               $p = new WP_HTML_Tag_Processor( '<div></div>' );
+               $p->next_tag();
+               $p->set_attribute( 'test', $attribute_value );
+
+               /*
+                * Testing the escaping is hard using tools that properly parse
+                * HTML because they might interpret the escaped values. It's hard
+                * with tools that don't understand HTML because they might get
+                * confused by improperly-escaped values.
+                *
+                * Since the input HTML is known, the test will do what looks like
+                * the opposite of what is expected to be done with this library.
+                * But by doing so, the test (a) has full control over the
+                * content and (b) looks at the raw values.
+                */
+               $match = null;
+               preg_match( '~^<div test=(.*)></div>$~', $p->get_updated_html(), $match );
+               list( , $actual_value ) = $match;
+
+               $this->assertSame( '"' . esc_attr( $attribute_value ) . '"', $actual_value, 'Entities were not properly escaped in the attribute value' );
+       }
+
+       /**
+        * Data provider.
+        *
+        * @return string[][].
+        */
+       public function data_set_attribute_prevents_xss() {
+               return array(
+                       array( '"' ),
+                       array( '&quot;' ),
+                       array( '&' ),
+                       array( '&amp;' ),
+                       array( '&euro;' ),
+                       array( "'" ),
+                       array( '<>' ),
+                       array( '&quot";' ),
+                       array( '" onclick="alert(\'1\');"><span onclick=""></span><script>alert("1")</script>' ),
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_set_attribute_with_a_non_existing_attribute_adds_a_new_attribute_to_the_markup() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->set_attribute( 'test-attribute', 'test-value' );
+
+               $this->assertSame(
+                       '<div test-attribute="test-value" id="first"><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not include attribute added via set_attribute()'
+               );
+               $this->assertSame(
+                       'test-value',
+                       $p->get_attribute( 'test-attribute' ),
+                       'get_attribute() (called after get_updated_html()) did not return attribute added via set_attribute()'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_returns_updated_values_before_they_are_applied() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->set_attribute( 'test-attribute', 'test-value' );
+
+               $this->assertSame(
+                       'test-value',
+                       $p->get_attribute( 'test-attribute' ),
+                       'get_attribute() (called before get_updated_html()) did not return attribute added via set_attribute()'
+               );
+               $this->assertSame(
+                       '<div test-attribute="test-value" id="first"><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not include attribute added via set_attribute()'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_returns_updated_values_before_they_are_applied_with_different_name_casing() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->set_attribute( 'test-ATTribute', 'test-value' );
+
+               $this->assertSame(
+                       'test-value',
+                       $p->get_attribute( 'test-attribute' ),
+                       'get_attribute() (called before get_updated_html()) did not return attribute added via set_attribute()'
+               );
+               $this->assertSame(
+                       '<div test-ATTribute="test-value" id="first"><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not include attribute added via set_attribute()'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_reflects_added_class_names_before_they_are_applied() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->add_class( 'my-class' );
+
+               $this->assertSame(
+                       'my-class',
+                       $p->get_attribute( 'class' ),
+                       'get_attribute() (called before get_updated_html()) did not return class name added via add_class()'
+               );
+               $this->assertSame(
+                       '<div class="my-class" id="first"><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not include class name added via add_class()'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_reflects_added_class_names_before_they_are_applied_and_retains_classes_from_previous_add_class_calls() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->add_class( 'my-class' );
+
+               $this->assertSame(
+                       'my-class',
+                       $p->get_attribute( 'class' ),
+                       'get_attribute() (called before get_updated_html()) did not return class name added via add_class()'
+               );
+
+               $p->add_class( 'my-other-class' );
+
+               $this->assertSame(
+                       'my-class my-other-class',
+                       $p->get_attribute( 'class' ),
+                       'get_attribute() (called before get_updated_html()) did not return class names added via subsequent add_class() calls'
+               );
+               $this->assertSame(
+                       '<div class="my-class my-other-class" id="first"><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not include class names added via subsequent add_class() calls'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_reflects_removed_attribute_before_it_is_applied() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->remove_attribute( 'id' );
+
+               $this->assertNull(
+                       $p->get_attribute( 'id' ),
+                       'get_attribute() (called before get_updated_html()) returned attribute that was removed by remove_attribute()'
+               );
+               $this->assertSame(
+                       '<div ><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML includes attribute that was removed by remove_attribute()'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_reflects_adding_and_then_removing_an_attribute_before_those_updates_are_applied() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->set_attribute( 'test-attribute', 'test-value' );
+               $p->remove_attribute( 'test-attribute' );
+
+               $this->assertNull(
+                       $p->get_attribute( 'test-attribute' ),
+                       'get_attribute() (called before get_updated_html()) returned attribute that was added via set_attribute() and then removed by remove_attribute()'
+               );
+               $this->assertSame(
+                       self::HTML_SIMPLE,
+                       $p->get_updated_html(),
+                       'Updated HTML includes attribute that was added via set_attribute() and then removed by remove_attribute()'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_reflects_setting_and_then_removing_an_existing_attribute_before_those_updates_are_applied() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->set_attribute( 'id', 'test-value' );
+               $p->remove_attribute( 'id' );
+
+               $this->assertNull(
+                       $p->get_attribute( 'id' ),
+                       'get_attribute() (called before get_updated_html()) returned attribute that was overwritten by set_attribute() and then removed by remove_attribute()'
+               );
+               $this->assertSame(
+                       '<div ><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML includes attribute that was overwritten by set_attribute() and then removed by remove_attribute()'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_reflects_removed_class_names_before_they_are_applied() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_WITH_CLASSES );
+               $p->next_tag();
+               $p->remove_class( 'with-border' );
+
+               $this->assertSame(
+                       'main',
+                       $p->get_attribute( 'class' ),
+                       'get_attribute() (called before get_updated_html()) returned the wrong attribute after calling remove_attribute()'
+               );
+               $this->assertSame(
+                       '<div class="main" id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML includes wrong attribute after calling remove_attribute()'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_reflects_setting_and_then_removing_a_class_name_before_those_updates_are_applied() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_WITH_CLASSES );
+               $p->next_tag();
+               $p->add_class( 'foo-class' );
+               $p->remove_class( 'foo-class' );
+
+               $this->assertSame(
+                       'main with-border',
+                       $p->get_attribute( 'class' ),
+                       'get_attribute() (called before get_updated_html()) returned class name that was added via add_class() and then removed by remove_class()'
+               );
+               $this->assertSame(
+                       self::HTML_WITH_CLASSES,
+                       $p->get_updated_html(),
+                       'Updated HTML includes class that was added via add_class() and then removed by remove_class()'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::get_attribute
+        */
+       public function test_get_attribute_reflects_duplicating_and_then_removing_an_existing_class_name_before_those_updates_are_applied() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_WITH_CLASSES );
+               $p->next_tag();
+               $p->add_class( 'with-border' );
+               $p->remove_class( 'with-border' );
+
+               $this->assertSame(
+                       'main',
+                       $p->get_attribute( 'class' ),
+                       'get_attribute() (called before get_updated_html()) returned class name that was duplicated via add_class() and then removed by remove_class()'
+               );
+               $this->assertSame(
+                       '<div class="main" id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML includes class that was duplicated via add_class() and then removed by remove_class()'
+               );
+       }
+
+       /**
+        * According to HTML spec, only the first instance of an attribute counts.
+        * The other ones are ignored.
+        *
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_update_first_attribute_when_duplicated_attributes_exist() {
+               $p = new WP_HTML_Tag_Processor( '<div id="update-me" id="ignored-id"><span id="second">Text</span></div>' );
+               $p->next_tag();
+               $p->set_attribute( 'id', 'updated-id' );
+
+               $this->assertSame(
+                       '<div id="updated-id" id="ignored-id"><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Proper (first) appearance of attribute was not updated when duplicates exist'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_set_attribute_with_an_existing_attribute_name_updates_its_value_in_the_markup() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->set_attribute( 'id', 'new-id' );
+               $this->assertSame(
+                       '<div id="new-id"><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Existing attribute was not updated'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::next_tag
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_next_tag_and_set_attribute_in_a_loop_update_all_tags_in_the_markup() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               while ( $p->next_tag() ) {
+                       $p->set_attribute( 'data-foo', 'bar' );
+               }
+
+               $this->assertSame(
+                       '<div data-foo="bar" id="first"><span data-foo="bar" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Not all tags were updated when looping with next_tag() and set_attribute()'
+               );
+       }
+
+       /**
+        * Removing an attribute that's listed many times, e.g. `<div id="a" id="b" />` should remove
+        * all its instances and output just `<div />`.
+        *
+        * Today, however, WP_HTML_Tag_Processor only removes the first such attribute. It seems like a corner case
+        * and introducing additional complexity to correctly handle this scenario doesn't seem to be worth it.
+        * Let's revisit if and when this becomes a problem.
+        *
+        * This test is in place to confirm this behavior, which while incorrect, is well-defined.
+        * A later fix introduced to the Tag Processor should update this test to reflect the
+        * wanted and correct behavior.
+        *
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::remove_attribute
+        */
+       public function test_remove_first_when_duplicated_attribute() {
+               $p = new WP_HTML_Tag_Processor( '<div id="update-me" id="ignored-id"><span id="second">Text</span></div>' );
+               $p->next_tag();
+               $p->remove_attribute( 'id' );
+
+               $this->assertSame(
+                       '<div  id="ignored-id"><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'First attribute (when duplicates exist) was not removed'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::remove_attribute
+        */
+       public function test_remove_attribute_with_an_existing_attribute_name_removes_it_from_the_markup() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->remove_attribute( 'id' );
+
+               $this->assertSame(
+                       '<div ><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Attribute was not removed'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::remove_attribute
+        */
+       public function test_remove_attribute_with_a_non_existing_attribute_name_does_not_change_the_markup() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->remove_attribute( 'no-such-attribute' );
+
+               $this->assertSame(
+                       self::HTML_SIMPLE,
+                       $p->get_updated_html(),
+                       'Content was changed when attempting to remove an attribute that did not exist'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::add_class
+        */
+       public function test_add_class_creates_a_class_attribute_when_there_is_none() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->add_class( 'foo-class' );
+
+               $this->assertSame(
+                       '<div class="foo-class" id="first"><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not include class name added via add_class()'
+               );
+               $this->assertSame(
+                       'foo-class',
+                       $p->get_attribute( 'class' ),
+                       "get_attribute( 'class' ) did not return class name added via add_class()"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::add_class
+        */
+       public function test_calling_add_class_twice_creates_a_class_attribute_with_both_class_names_when_there_is_no_class_attribute() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->add_class( 'foo-class' );
+               $p->add_class( 'bar-class' );
+
+               $this->assertSame(
+                       '<div class="foo-class bar-class" id="first"><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not include class names added via subsequent add_class() calls'
+               );
+               $this->assertSame(
+                       'foo-class bar-class',
+                       $p->get_attribute( 'class' ),
+                       "get_attribute( 'class' ) did not return class names added via subsequent add_class() calls"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::remove_class
+        */
+       public function test_remove_class_does_not_change_the_markup_when_there_is_no_class_attribute() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->remove_class( 'foo-class' );
+
+               $this->assertSame(
+                       self::HTML_SIMPLE,
+                       $p->get_updated_html(),
+                       'Updated HTML includes class name that was removed by remove_class()'
+               );
+               $this->assertNull(
+                       $p->get_attribute( 'class' ),
+                       "get_attribute( 'class' ) did not return null for class name that was removed by remove_class()"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::add_class
+        */
+       public function test_add_class_appends_class_names_to_the_existing_class_attribute_when_one_already_exists() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_WITH_CLASSES );
+               $p->next_tag();
+               $p->add_class( 'foo-class' );
+               $p->add_class( 'bar-class' );
+
+               $this->assertSame(
+                       '<div class="main with-border foo-class bar-class" id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not reflect class names added to existing class attribute via subsequent add_class() calls'
+               );
+               $this->assertSame(
+                       'main with-border foo-class bar-class',
+                       $p->get_attribute( 'class' ),
+                       "get_attribute( 'class' ) does not reflect class names added to existing class attribute via subsequent add_class() calls"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::remove_class
+        */
+       public function test_remove_class_removes_a_single_class_from_the_class_attribute_when_one_exists() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_WITH_CLASSES );
+               $p->next_tag();
+               $p->remove_class( 'main' );
+
+               $this->assertSame(
+                       '<div class=" with-border" id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not reflect class name removed from existing class attribute via remove_class()'
+               );
+               $this->assertSame(
+                       ' with-border',
+                       $p->get_attribute( 'class' ),
+                       "get_attribute( 'class' ) does not reflect class name removed from existing class attribute via remove_class()"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::remove_class
+        */
+       public function test_calling_remove_class_with_all_listed_class_names_removes_the_existing_class_attribute_from_the_markup() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_WITH_CLASSES );
+               $p->next_tag();
+               $p->remove_class( 'main' );
+               $p->remove_class( 'with-border' );
+
+               $this->assertSame(
+                       '<div  id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not reflect class attribute removed via subesequent remove_class() calls'
+               );
+               $this->assertNull(
+                       $p->get_attribute( 'class' ),
+                       "get_attribute( 'class' ) did not return null for class attribute removed via subesequent remove_class() calls"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::add_class
+        */
+       public function test_add_class_does_not_add_duplicate_class_names() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_WITH_CLASSES );
+               $p->next_tag();
+               $p->add_class( 'with-border' );
+
+               $this->assertSame(
+                       '<div class="main with-border" id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not reflect deduplicated class name added via add_class()'
+               );
+               $this->assertSame(
+                       'main with-border',
+                       $p->get_attribute( 'class' ),
+                       "get_attribute( 'class' ) does not reflect deduplicated class name added via add_class()"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::add_class
+        */
+       public function test_add_class_preserves_class_name_order_when_a_duplicate_class_name_is_added() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_WITH_CLASSES );
+               $p->next_tag();
+               $p->add_class( 'main' );
+
+               $this->assertSame(
+                       '<div class="main with-border" id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not reflect class name order after adding duplicated class name via add_class()'
+               );
+               $this->assertSame(
+                       'main with-border',
+                       $p->get_attribute( 'class' ),
+                       "get_attribute( 'class' ) does not reflect class name order after adding duplicated class name added via add_class()"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::add_class
+        */
+       public function test_add_class_when_there_is_a_class_attribute_with_excessive_whitespaces() {
+               $p = new WP_HTML_Tag_Processor(
+                       '<div class="   main   with-border   " id="first"><span class="not-main bold with-border" id="second">Text</span></div>'
+               );
+               $p->next_tag();
+               $p->add_class( 'foo-class' );
+
+               $this->assertSame(
+                       '<div class="   main   with-border foo-class" id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not reflect existing excessive whitespace after adding class name via add_class()'
+               );
+               $this->assertSame(
+                       '   main   with-border foo-class',
+                       $p->get_attribute( 'class' ),
+                       "get_attribute( 'class' ) does not reflect existing excessive whitespace after adding class name via add_class()"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::remove_class
+        */
+       public function test_remove_class_preserves_whitespaces_when_there_is_a_class_attribute_with_excessive_whitespaces() {
+               $p = new WP_HTML_Tag_Processor(
+                       '<div class="   main   with-border   " id="first"><span class="not-main bold with-border" id="second">Text</span></div>'
+               );
+               $p->next_tag();
+               $p->remove_class( 'with-border' );
+
+               $this->assertSame(
+                       '<div class="   main" id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not reflect existing excessive whitespace after removing class name via remove_class()'
+               );
+               $this->assertSame(
+                       '   main',
+                       $p->get_attribute( 'class' ),
+                       "get_attribute( 'class' ) does not reflect existing excessive whitespace after removing class name via removing_class()"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::remove_class
+        */
+       public function test_removing_all_classes_removes_the_existing_class_attribute_from_the_markup_even_when_excessive_whitespaces_are_present() {
+               $p = new WP_HTML_Tag_Processor(
+                       '<div class="   main   with-border   " id="first"><span class="not-main bold with-border" id="second">Text</span></div>'
+               );
+               $p->next_tag();
+               $p->remove_class( 'main' );
+               $p->remove_class( 'with-border' );
+               $this->assertSame(
+                       '<div  id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Updated HTML does not reflect removed class attribute after removing all class names via remove_class()'
+               );
+               $this->assertNull(
+                       $p->get_attribute( 'class' ),
+                       "get_attribute( 'class' ) did not return null after removing all class names via remove_class()"
+               );
+       }
+
+       /**
+        * When add_class( $different_value ) is called _after_ set_attribute( 'class', $value ), the
+        * final class name should be "$value $different_value". In other words, the `add_class` call
+        * should append its class to the one(s) set by `set_attribute`. When `add_class( $different_value )`
+        * is called _before_ `set_attribute( 'class', $value )`, however, the final class name should be
+        * "$value" instead, as any direct updates to the `class` attribute supersede any changes enqueued
+        * via the class builder methods.
+        *
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::add_class
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_set_attribute_takes_priority_over_add_class() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_WITH_CLASSES );
+               $p->next_tag();
+               $p->add_class( 'add_class' );
+               $p->set_attribute( 'class', 'set_attribute' );
+               $this->assertSame(
+                       '<div class="set_attribute" id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       "Calling get_updated_html after updating first tag's attributes did not return the expected HTML"
+               );
+               $this->assertSame(
+                       'set_attribute',
+                       $p->get_attribute( 'class' ),
+                       "Calling get_attribute after updating first tag's attributes did not return the expected class name"
+               );
+
+               $p = new WP_HTML_Tag_Processor( self::HTML_WITH_CLASSES );
+               $p->next_tag();
+               $p->set_attribute( 'class', 'set_attribute' );
+               $p->add_class( 'add_class' );
+               $this->assertSame(
+                       '<div class="set_attribute add_class" id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       "Calling get_updated_html after updating first tag's attributes did not return the expected HTML"
+               );
+               $this->assertSame(
+                       'set_attribute add_class',
+                       $p->get_attribute( 'class' ),
+                       "Calling get_attribute after updating first tag's attributes did not return the expected class name"
+               );
+       }
+
+       /**
+        * When add_class( $different_value ) is called _after_ set_attribute( 'class', $value ), the
+        * final class name should be "$value $different_value". In other words, the `add_class` call
+        * should append its class to the one(s) set by `set_attribute`. When `add_class( $different_value )`
+        * is called _before_ `set_attribute( 'class', $value )`, however, the final class name should be
+        * "$value" instead, as any direct updates to the `class` attribute supersede any changes enqueued
+        * via the class builder methods.
+        *
+        * This is still true when reading enqueued updates before calling `get_updated_html()`.
+        *
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::add_class
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_set_attribute_takes_priority_over_add_class_even_before_updating() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_WITH_CLASSES );
+               $p->next_tag();
+               $p->add_class( 'add_class' );
+               $p->set_attribute( 'class', 'set_attribute' );
+               $this->assertSame(
+                       'set_attribute',
+                       $p->get_attribute( 'class' ),
+                       "Calling get_attribute after updating first tag's attributes did not return the expected class name"
+               );
+               $this->assertSame(
+                       '<div class="set_attribute" id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       "Calling get_updated_html after updating first tag's attributes did not return the expected HTML"
+               );
+
+               $p = new WP_HTML_Tag_Processor( self::HTML_WITH_CLASSES );
+               $p->next_tag();
+               $p->set_attribute( 'class', 'set_attribute' );
+               $p->add_class( 'add_class' );
+               $this->assertSame(
+                       'set_attribute add_class',
+                       $p->get_attribute( 'class' ),
+                       "Calling get_attribute after updating first tag's attributes did not return the expected class name"
+               );
+               $this->assertSame(
+                       '<div class="set_attribute add_class" id="first"><span class="not-main bold with-border" id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       "Calling get_updated_html after updating first tag's attributes did not return the expected HTML"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::add_class
+        */
+       public function test_add_class_overrides_boolean_class_attribute() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->set_attribute( 'class', true );
+               $p->add_class( 'add_class' );
+               $this->assertSame(
+                       '<div class="add_class" id="first"><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       "Updated HTML doesn't reflect class added via add_class that was originally set as boolean attribute"
+               );
+               $this->assertSame(
+                       'add_class',
+                       $p->get_attribute( 'class' ),
+                       "get_attribute (called after get_updated_html()) doesn't reflect class added via add_class that was originally set as boolean attribute"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::add_class
+        */
+       public function test_add_class_overrides_boolean_class_attribute_even_before_updating() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_SIMPLE );
+               $p->next_tag();
+               $p->set_attribute( 'class', true );
+               $p->add_class( 'add_class' );
+               $this->assertSame(
+                       'add_class',
+                       $p->get_attribute( 'class' ),
+                       "get_attribute (called before get_updated_html()) doesn't reflect class added via add_class that was originally set as boolean attribute"
+               );
+               $this->assertSame(
+                       '<div class="add_class" id="first"><span id="second">Text</span></div>',
+                       $p->get_updated_html(),
+                       "Updated HTML doesn't reflect class added via add_class that was originally set as boolean attribute"
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        * @covers WP_HTML_Tag_Processor::remove_attribute
+        * @covers WP_HTML_Tag_Processor::add_class
+        * @covers WP_HTML_Tag_Processor::remove_class
+        * @covers WP_HTML_Tag_Processor::get_updated_html
+        */
+       public function test_advanced_use_case() {
+               $input = <<<HTML
+<div selected class="merge-message" checked>
+       <div class="select-menu d-inline-block">
+               <div checked class="BtnGroup MixedCaseHTML position-relative" />
+               <div checked class="BtnGroup MixedCaseHTML position-relative">
+                       <button type="button" class="merge-box-button btn-group-merge rounded-left-2 btn  BtnGroup-item js-details-target hx_create-pr-button" aria-expanded="false" data-details-container=".js-merge-pr" disabled="">
+                         Merge pull request
+                       </button>
+
+                       <button type="button" class="merge-box-button btn-group-squash rounded-left-2 btn  BtnGroup-item js-details-target hx_create-pr-button" aria-expanded="false" data-details-container=".js-merge-pr" disabled="">
+                         Squash and merge
+                       </button>
+
+                       <button type="button" class="merge-box-button btn-group-rebase rounded-left-2 btn  BtnGroup-item js-details-target hx_create-pr-button" aria-expanded="false" data-details-container=".js-merge-pr" disabled="">
+                         Rebase and merge
+                       </button>
+
+                       <button aria-label="Select merge method" disabled="disabled" type="button" data-view-component="true" class="select-menu-button btn BtnGroup-item"></button>
+               </div>
+       </div>
+</div>
+HTML;
+
+               $expected_output = <<<HTML
+<div data-details="{ &quot;key&quot;: &quot;value&quot; }" selected class="merge-message is-processed" checked>
+       <div class="select-menu d-inline-block">
+               <div checked class=" MixedCaseHTML position-relative button-group Another-Mixed-Case" />
+               <div checked class=" MixedCaseHTML position-relative button-group Another-Mixed-Case">
+                       <button type="button" class="merge-box-button btn-group-merge rounded-left-2 btn  BtnGroup-item js-details-target hx_create-pr-button" aria-expanded="false" data-details-container=".js-merge-pr" disabled="">
+                         Merge pull request
+                       </button>
+
+                       <button type="button" class="merge-box-button btn-group-squash rounded-left-2 btn  BtnGroup-item js-details-target hx_create-pr-button" aria-expanded="false" data-details-container=".js-merge-pr" disabled="">
+                         Squash and merge
+                       </button>
+
+                       <button type="button"  aria-expanded="false" data-details-container=".js-merge-pr" disabled="">
+                         Rebase and merge
+                       </button>
+
+                       <button aria-label="Select merge method" disabled="disabled" type="button" data-view-component="true" class="select-menu-button btn BtnGroup-item"></button>
+               </div>
+       </div>
+</div>
+HTML;
+
+               $p = new WP_HTML_Tag_Processor( $input );
+               $this->assertTrue( $p->next_tag( 'div' ), 'Querying an existing tag did not return true' );
+               $p->set_attribute( 'data-details', '{ "key": "value" }' );
+               $p->add_class( 'is-processed' );
+               $this->assertTrue(
+                       $p->next_tag(
+                               array(
+                                       'tag_name'   => 'div',
+                                       'class_name' => 'BtnGroup',
+                               )
+                       ),
+                       'Querying an existing tag did not return true'
+               );
+               $p->remove_class( 'BtnGroup' );
+               $p->add_class( 'button-group' );
+               $p->add_class( 'Another-Mixed-Case' );
+               $this->assertTrue(
+                       $p->next_tag(
+                               array(
+                                       'tag_name'   => 'div',
+                                       'class_name' => 'BtnGroup',
+                               )
+                       ),
+                       'Querying an existing tag did not return true'
+               );
+               $p->remove_class( 'BtnGroup' );
+               $p->add_class( 'button-group' );
+               $p->add_class( 'Another-Mixed-Case' );
+               $this->assertTrue(
+                       $p->next_tag(
+                               array(
+                                       'tag_name'     => 'button',
+                                       'class_name'   => 'btn',
+                                       'match_offset' => 3,
+                               )
+                       ),
+                       'Querying an existing tag did not return true'
+               );
+               $p->remove_attribute( 'class' );
+               $this->assertFalse( $p->next_tag( 'non-existent' ), 'Querying a non-existing tag did not return false' );
+               $p->set_attribute( 'class', 'test' );
+               $this->assertSame( $expected_output, $p->get_updated_html(), 'Calling get_updated_html after updating the attributes did not return the expected HTML' );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::next_tag
+        */
+       public function test_correctly_parses_html_attributes_wrapped_in_single_quotation_marks() {
+               $p = new WP_HTML_Tag_Processor(
+                       '<div id=\'first\'><span id=\'second\'>Text</span></div>'
+               );
+               $p->next_tag(
+                       array(
+                               'tag_name' => 'div',
+                               'id'       => 'first',
+                       )
+               );
+               $p->remove_attribute( 'id' );
+               $p->next_tag(
+                       array(
+                               'tag_name' => 'span',
+                               'id'       => 'second',
+                       )
+               );
+               $p->set_attribute( 'id', 'single-quote' );
+               $this->assertSame(
+                       '<div ><span id="single-quote">Text</span></div>',
+                       $p->get_updated_html(),
+                       'Did not remove single-quoted attribute'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_set_attribute_with_value_equal_to_true_adds_a_boolean_html_attribute_with_implicit_value() {
+               $p = new WP_HTML_Tag_Processor(
+                       '<form action="/action_page.php"><input type="checkbox" name="vehicle" value="Bike"><label for="vehicle">I have a bike</label></form>'
+               );
+               $p->next_tag( 'input' );
+               $p->set_attribute( 'checked', true );
+               $this->assertSame(
+                       '<form action="/action_page.php"><input checked type="checkbox" name="vehicle" value="Bike"><label for="vehicle">I have a bike</label></form>',
+                       $p->get_updated_html(),
+                       'Did not add "checked" as an expected boolean attribute'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_setting_a_boolean_attribute_to_false_removes_it_from_the_markup() {
+               $p = new WP_HTML_Tag_Processor(
+                       '<form action="/action_page.php"><input checked type="checkbox" name="vehicle" value="Bike"><label for="vehicle">I have a bike</label></form>'
+               );
+               $p->next_tag( 'input' );
+               $p->set_attribute( 'checked', false );
+               $this->assertSame(
+                       '<form action="/action_page.php"><input  type="checkbox" name="vehicle" value="Bike"><label for="vehicle">I have a bike</label></form>',
+                       $p->get_updated_html(),
+                       'Did not remove boolean attribute when set to false'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_setting_a_missing_attribute_to_false_does_not_change_the_markup() {
+               $html_input = '<form action="/action_page.php"><input type="checkbox" name="vehicle" value="Bike"><label for="vehicle">I have a bike</label></form>';
+               $p          = new WP_HTML_Tag_Processor( $html_input );
+               $p->next_tag( 'input' );
+               $p->set_attribute( 'checked', false );
+               $this->assertSame(
+                       $html_input,
+                       $p->get_updated_html(),
+                       'Changed the markup unexpectedly when setting a non-existing attribute to false'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_setting_a_boolean_attribute_to_a_string_value_adds_explicit_value_to_the_markup() {
+               $p = new WP_HTML_Tag_Processor(
+                       '<form action="/action_page.php"><input checked type="checkbox" name="vehicle" value="Bike"><label for="vehicle">I have a bike</label></form>'
+               );
+               $p->next_tag( 'input' );
+               $p->set_attribute( 'checked', 'checked' );
+               $this->assertSame(
+                       '<form action="/action_page.php"><input checked="checked" type="checkbox" name="vehicle" value="Bike"><label for="vehicle">I have a bike</label></form>',
+                       $p->get_updated_html(),
+                       'Did not add string value to existing boolean attribute'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::next_tag
+        */
+       public function test_unclosed_script_tag_should_not_cause_an_infinite_loop() {
+               $p = new WP_HTML_Tag_Processor( '<script>' );
+               $p->next_tag();
+               $this->assertSame( 'SCRIPT', $p->get_tag(), 'Did not find script tag' );
+               $p->next_tag();
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::next_tag
+        *
+        * @dataProvider data_next_tag_ignores_script_tag_contents
+        *
+        * @param string $script_then_div HTML to test.
+        */
+       public function test_next_tag_ignores_script_tag_contents( $script_then_div ) {
+               $p = new WP_HTML_Tag_Processor( $script_then_div );
+               $p->next_tag();
+               $this->assertSame( 'SCRIPT', $p->get_tag(), 'The first found tag was not "script"' );
+               $p->next_tag();
+               $this->assertSame( 'DIV', $p->get_tag(), 'The second found tag was not "div"' );
+       }
+
+       /**
+        * Data provider.
+        *
+        * @return array[].
+        */
+       public function data_next_tag_ignores_script_tag_contents() {
+               return array(
+                       'Simple script tag'                          => array(
+                               '<script><span class="d-none d-md-inline">Back to notifications</span></script><div></div>',
+                       ),
+
+                       'Simple uppercase script tag'                => array(
+                               '<script><span class="d-none d-md-inline">Back to notifications</span></SCRIPT><div></div>',
+                       ),
+
+                       'Script with a comment opener inside should end at the next script tag closer (dash dash escaped state)' => array(
+                               '<script class="d-md-none"><!--</script><div></div>-->',
+                       ),
+
+                       'Script with a comment opener and a script tag opener inside should end two script tag closer later (double escaped state)' => array(
+                               '<script class="d-md-none"><!--<script><span1></script><span2></span2></script><div></div>-->',
+                       ),
+
+                       'Double escaped script with a tricky opener' => array(
+                               '<script class="d-md-none"><!--<script attr="</script>"></script>"><div></div>',
+                       ),
+
+                       'Double escaped script with a tricky closer' => array(
+                               '<script class="d-md-none"><!--<script><span></script attr="</script>"><div></div>',
+                       ),
+
+                       'Double escaped, then escaped, then double escaped' => array(
+                               '<script class="d-md-none"><!--<script></script><script></script><span></span></script><div></div>',
+                       ),
+
+                       'Script with a commented a script tag opener inside should at the next tag closer (dash dash escaped state)' => array(
+                               '<script class="d-md-none"><!--<script>--><span></script><div></div>-->',
+                       ),
+
+                       'Script closer with another script tag in closer attributes' => array(
+                               '<script><span class="d-none d-md-inline">Back to notifications</title</span></script <script><div></div>',
+                       ),
+
+                       'Script closer with attributes'              => array(
+                               '<script class="d-md-none"><span class="d-none d-md-inline">Back to notifications</span></script id="test"><div></div>',
+                       ),
+
+                       'Script opener with title closer inside'     => array(
+                               '<script class="d-md-none"></title></script><div></div>',
+                       ),
+
+                       'Complex script with many parsing states'    => array(
+                               '<script class="d-md-none"><!--<script>--><scRipt><span><!--<span><Script</script>--></scripT><div></div>-->',
+                       ),
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::next_tag
+        *
+        * @dataProvider data_next_tag_ignores_contents_of_rcdata_tag
+        *
+        * @param string $rcdata_then_div HTML with RCDATA before a DIV.
+        * @param string $rcdata_tag      RCDATA tag.
+        */
+       public function test_next_tag_ignores_contents_of_rcdata_tag( $rcdata_then_div, $rcdata_tag ) {
+               $p = new WP_HTML_Tag_Processor( $rcdata_then_div );
+               $p->next_tag();
+               $this->assertSame( $rcdata_tag, $p->get_tag(), "The first found tag was not '$rcdata_tag'" );
+               $p->next_tag();
+               $this->assertSame( 'DIV', $p->get_tag(), "The second found tag was not 'div'" );
+       }
+
+       /**
+        * Data provider.
+        *
+        * @return array[]
+        */
+       public function data_next_tag_ignores_contents_of_rcdata_tag() {
+               return array(
+                       'simple textarea'                          => array(
+                               'rcdata_then_div' => '<textarea><span class="d-none d-md-inline">Back to notifications</span></textarea><div></div>',
+                               'rcdata_tag'      => 'TEXTAREA',
+                       ),
+                       'simple title'                             => array(
+                               'rcdata_then_div' => '<title><span class="d-none d-md-inline">Back to notifications</title</span></title><div></div>',
+                               'rcdata_tag'      => 'TITLE',
+                       ),
+                       'comment opener inside a textarea tag should be ignored' => array(
+                               'rcdata_then_div' => '<textarea class="d-md-none"><!--</textarea><div></div>-->',
+                               'rcdata_tag'      => 'TEXTAREA',
+                       ),
+                       'textarea closer with another textarea tag in closer attributes' => array(
+                               'rcdata_then_div' => '<textarea><span class="d-none d-md-inline">Back to notifications</title</span></textarea <textarea><div></div>',
+                               'rcdata_tag'      => 'TEXTAREA',
+                       ),
+                       'textarea closer with attributes'          => array(
+                               'rcdata_then_div' => '<textarea class="d-md-none"><span class="d-none d-md-inline">Back to notifications</span></textarea id="test"><div></div>',
+                               'rcdata_tag'      => 'TEXTAREA',
+                       ),
+                       'textarea opener with title closer inside' => array(
+                               'rcdata_then_div' => '<textarea class="d-md-none"></title></textarea><div></div>',
+                               'rcdata_tag'      => 'TEXTAREA',
+                       ),
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::next_tag
+        *
+        * @dataProvider data_skips_contents_of_script_and_rcdata_regions
+        *
+        * @param $input_html HTML with multiple divs, one of which carries the "target" attribute.
+        */
+       public function test_skips_contents_of_script_and_rcdata_regions( $input_html ) {
+               $p = new WP_HTML_Tag_Processor( $input_html );
+               $p->next_tag( 'div' );
+
+               $this->assertTrue(
+                       $p->get_attribute( 'target' ),
+                       'Did not properly skip over script and rcdata regions; incorrectly found tags inside'
+               );
+       }
+
+       /**
+        * Data provider
+        *
+        * @return array[]
+        */
+       public function data_skips_contents_of_script_and_rcdata_regions() {
+               return array(
+                       'Balanced SCRIPT tags'                => array( '<script>console.log("<div>");</script><div target><div>' ),
+                       'Unexpected SCRIPT closer after DIV'  => array( 'console.log("<div target>")</script><div><div>' ),
+                       'Unexpected SCRIPT closer before DIV' => array( 'console.log("<span>")</script><div target><div>' ),
+                       'Missing SCRIPT closer'               => array( '<script>console.log("<div>");<div><div></script><div target>' ),
+                       'TITLE before DIV'                    => array( '<title><div></title><div target><div>' ),
+                       'SCRIPT inside TITLE'                 => array( '<title><script><div></title><div target><div></script><div>' ),
+                       'TITLE in TEXTAREA'                   => array( '<textarea><div><title><div></textarea><div target></title><div>' ),
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::next_tag
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_can_query_and_update_wrongly_nested_tags() {
+               $p = new WP_HTML_Tag_Processor(
+                       '<span>123<p>456</span>789</p>'
+               );
+               $p->next_tag( 'span' );
+               $p->set_attribute( 'class', 'span-class' );
+               $p->next_tag( 'p' );
+               $p->set_attribute( 'class', 'p-class' );
+               $this->assertSame(
+                       '<span class="span-class">123<p class="p-class">456</span>789</p>',
+                       $p->get_updated_html(),
+                       'Did not find overlapping p tag'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::next_tag
+        * @covers WP_HTML_Tag_Processor::remove_attribute
+        */
+       public function test_removing_specific_attributes_in_malformed_html() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_MALFORMED );
+               $p->next_tag( 'span' );
+               $p->remove_attribute( 'Notifications<' );
+               $this->assertSame(
+                       '<div><span class="d-md-none" /span><span class="d-none d-md-inline">Back to notifications</span></div>',
+                       $p->get_updated_html(),
+                       'Did not remove "Notifications<" attribute in malformed input'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        */
+       public function test_updating_specific_attributes_in_malformed_html() {
+               $p = new WP_HTML_Tag_Processor( self::HTML_MALFORMED );
+               $p->next_tag( 'span' );
+               $p->set_attribute( 'id', 'first' );
+               $p->next_tag( 'span' );
+               $p->set_attribute( 'id', 'second' );
+               $this->assertSame(
+                       '<div><span id="first" class="d-md-none" Notifications</span><span id="second" class="d-none d-md-inline">Back to notifications</span></div>',
+                       $p->get_updated_html(),
+                       'Did not add id attributes properly to malformed input'
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::add_class
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        *
+        * @dataProvider data_updating_attributes
+        *
+        * @param string $html     HTML to process.
+        * @param string $expected Expected updated HTML.
+        */
+       public function test_updating_attributes( $html, $expected ) {
+               $p = new WP_HTML_Tag_Processor( $html );
+               $p->next_tag();
+               $p->set_attribute( 'foo', 'bar' );
+               $p->add_class( 'firstTag' );
+               $p->next_tag();
+               $p->add_class( 'secondTag' );
+
+               $this->assertSame(
+                       $expected,
+                       $p->get_updated_html(),
+                       'Did not properly add attributes and class names'
+               );
+       }
+
+       /**
+        * Data provider.
+        *
+        * @return array[]
+        */
+       public function data_updating_attributes() {
+               return array(
+                       'tags inside of a comment' => array(
+                               'input'    => '<!-- this is a comment. no <strong>tags</strong> allowed --><span>test</span>',
+                               'expected' => '<!-- this is a comment. no <strong>tags</strong> allowed --><span class="firstTag" foo="bar">test</span>',
+                       ),
+                       'does not parse <3'        => array(
+                               'input'    => '<3 is a heart but <t3> is a tag.<span>test</span>',
+                               'expected' => '<3 is a heart but <t3 class="firstTag" foo="bar"> is a tag.<span class="secondTag">test</span>',
+                       ),
+                       'does not parse <*'        => array(
+                               'input'    => 'The applicative operator <* works well in Haskell; is what?<span>test</span>',
+                               'expected' => 'The applicative operator <* works well in Haskell; is what?<span class="firstTag" foo="bar">test</span>',
+                       ),
+                       '</> in content'           => array(
+                               'input'    => '</><span>test</span>',
+                               'expected' => '</><span class="firstTag" foo="bar">test</span>',
+                       ),
+                       'custom asdf attribute'    => array(
+                               'input'    => '<hr asdf="test"><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" asdf="test"><span class="secondTag">test</span>',
+                       ),
+                       'custom data-* attribute'  => array(
+                               'input'    => '<div data-foo="bar"><p>Some content for a <span>test</span></p></div>',
+                               'expected' => '<div class="firstTag" foo="bar" data-foo="bar"><p class="secondTag">Some content for a <span>test</span></p></div>',
+                       ),
+                       'tag inside of CDATA'      => array(
+                               'input'    => '<![CDATA[This <is> a <strong id="yes">HTML Tag</strong>]]><span>test</span>',
+                               'expected' => '<![CDATA[This <is> a <strong id="yes">HTML Tag</strong>]]><span class="firstTag" foo="bar">test</span>',
+                       ),
+               );
+       }
+
+       /**
+        * @ticket 56299
+        *
+        * @covers WP_HTML_Tag_Processor::add_class
+        * @covers WP_HTML_Tag_Processor::set_attribute
+        *
+        * @dataProvider data_updating_attributes_in_malformed_html
+        *
+        * @param string $html     HTML to process.
+        * @param string $expected Expected updated HTML.
+        */
+       public function test_updating_attributes_in_malformed_html( $html, $expected ) {
+               $p = new WP_HTML_Tag_Processor( $html );
+               $p->next_tag();
+               $p->set_attribute( 'foo', 'bar' );
+               $p->add_class( 'firstTag' );
+               $p->next_tag();
+               $p->add_class( 'secondTag' );
+
+               $this->assertSame(
+                       $expected,
+                       $p->get_updated_html(),
+                       'Did not properly update attributes and classnames given malformed input'
+               );
+       }
+
+       /**
+        * Data provider.
+        *
+        * @return array[]
+        */
+       public function data_updating_attributes_in_malformed_html() {
+               $null_byte = chr( 0 );
+
+               return array(
+                       'Invalid entity inside attribute value'        => array(
+                               'input'    => '<img src="https://s0.wp.com/i/atat.png" title="&; First &lt;title&gt; is &notit;" TITLE="second title" title="An Imperial &imperial; AT-AT"><span>test</span>',
+                               'expected' => '<img class="firstTag" foo="bar" src="https://s0.wp.com/i/atat.png" title="&; First &lt;title&gt; is &notit;" TITLE="second title" title="An Imperial &imperial; AT-AT"><span class="secondTag">test</span>',
+                       ),
+                       'HTML tag opening inside attribute value'      => array(
+                               'input'    => '<pre id="<code" class="wp-block-code <code is poetry&gt;"><code>This &lt;is> a &lt;strong is="true">thing.</code></pre><span>test</span>',
+                               'expected' => '<pre foo="bar" id="<code" class="wp-block-code &lt;code is poetry&gt; firstTag"><code class="secondTag">This &lt;is> a &lt;strong is="true">thing.</code></pre><span>test</span>',
+                       ),
+                       'HTML tag brackets in attribute values and data markup' => array(
+                               'input'    => '<pre id="<code-&gt;-block-&gt;" class="wp-block-code <code is poetry&gt;"><code>This &lt;is> a &lt;strong is="true">thing.</code></pre><span>test</span>',
+                               'expected' => '<pre foo="bar" id="<code-&gt;-block-&gt;" class="wp-block-code &lt;code is poetry&gt; firstTag"><code class="secondTag">This &lt;is> a &lt;strong is="true">thing.</code></pre><span>test</span>',
+                       ),
+                       'Single and double quotes in attribute value'  => array(
+                               'input'    => '<p title="Demonstrating how to use single quote (\') and double quote (&quot;)"><span>test</span>',
+                               'expected' => '<p class="firstTag" foo="bar" title="Demonstrating how to use single quote (\') and double quote (&quot;)"><span class="secondTag">test</span>',
+                       ),
+                       'Unquoted attribute values'                    => array(
+                               'input'    => '<hr a=1 a=2 a=3 a=5 /><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" a=1 a=2 a=3 a=5 /><span class="secondTag">test</span>',
+                       ),
+                       'Double-quotes escaped in double-quote attribute value' => array(
+                               'input'    => '<hr title="This is a &quot;double-quote&quot;"><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" title="This is a &quot;double-quote&quot;"><span class="secondTag">test</span>',
+                       ),
+                       'Unquoted attribute value'                     => array(
+                               'input'    => '<hr id=code><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" id=code><span class="secondTag">test</span>',
+                       ),
+                       'Unquoted attribute value with tag-like value' => array(
+                               'input'    => '<hr id=    <code> ><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" id=     <code> ><span class="secondTag">test</span>',
+                       ),
+                       'Unquoted attribute value with tag-like value followed by tag-like data' => array(
+                               'input'    => '<hr id=code>><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" id=code>><span class="secondTag">test</span>',
+                       ),
+                       'id=&quo;code'                                 => array(
+                               'input'    => '<hr id=&quo;code><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" id=&quo;code><span class="secondTag">test</span>',
+                       ),
+                       'id/test=5'                                    => array(
+                               'input'    => '<hr id/test=5><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" id/test=5><span class="secondTag">test</span>',
+                       ),
+                       '<hr> as the id value'                         => array(
+                               'input'    => '<hr title="<hr>"><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" title="<hr>"><span class="secondTag">test</span>',
+                       ),
+                       'id=>code'                                     => array(
+                               'input'    => '<hr id=>code><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" id=>code><span class="secondTag">test</span>',
+                       ),
+                       'id"quo="test"'                                => array(
+                               'input'    => '<hr id"quo="test"><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" id"quo="test"><span class="secondTag">test</span>',
+                       ),
+                       'id without double quotation marks around null byte' => array(
+                               'input'    => '<hr id' . $null_byte . 'zero="test"><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" id' . $null_byte . 'zero="test"><span class="secondTag">test</span>',
+                       ),
+                       'Unexpected > before an attribute'             => array(
+                               'input'    => '<hr >id="test"><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" >id="test"><span class="secondTag">test</span>',
+                       ),
+                       'Unexpected = before an attribute'             => array(
+                               'input'    => '<hr =id="test"><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" =id="test"><span class="secondTag">test</span>',
+                       ),
+                       'Unexpected === before an attribute'           => array(
+                               'input'    => '<hr ===name="value"><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" ===name="value"><span class="secondTag">test</span>',
+                       ),
+                       'Missing closing data-tag tag'                 => array(
+                               'input'    => 'The applicative operator <* works well in Haskell; <data-tag> is what?<span>test</span>',
+                               'expected' => 'The applicative operator <* works well in Haskell; <data-tag class="firstTag" foo="bar"> is what?<span class="secondTag">test</span>',
+                       ),
+                       'Missing closing t3 tag'                       => array(
+                               'input'    => '<3 is a heart but <t3> is a tag.<span>test</span>',
+                               'expected' => '<3 is a heart but <t3 class="firstTag" foo="bar"> is a tag.<span class="secondTag">test</span>',
+                       ),
+                       'invalid comment opening tag'                  => array(
+                               'input'    => '<?comment --><span>test</span>',
+                               'expected' => '<?comment --><span class="firstTag" foo="bar">test</span>',
+                       ),
+                       '=asdf as attribute name'                      => array(
+                               'input'    => '<hr =asdf="tes"><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" =asdf="tes"><span class="secondTag">test</span>',
+                       ),
+                       '== as attribute name with value'              => array(
+                               'input'    => '<hr ==="test"><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" ==="test"><span class="secondTag">test</span>',
+                       ),
+                       '=5 as attribute'                              => array(
+                               'input'    => '<hr =5><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" =5><span class="secondTag">test</span>',
+                       ),
+                       '= as attribute'                               => array(
+                               'input'    => '<hr =><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" =><span class="secondTag">test</span>',
+                       ),
+                       '== as attribute'                              => array(
+                               'input'    => '<hr ==><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" ==><span class="secondTag">test</span>',
+                       ),
+                       '=== as attribute'                             => array(
+                               'input'    => '<hr ===><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" ===><span class="secondTag">test</span>',
+                       ),
+                       'unsupported disabled attribute'               => array(
+                               'input'    => '<hr disabled><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" disabled><span class="secondTag">test</span>',
+                       ),
+                       'malformed custom attributes'                  => array(
+                               'input'    => '<hr a"sdf="test"><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" a"sdf="test"><span class="secondTag">test</span>',
+                       ),
+                       'Multiple unclosed tags treated as a single tag' => array(
+                               'input'    => <<<HTML
+                                       <hr id=">"code
+                                       <hr id="value>"code
+                                       <hr id="/>"code
+                                       <hr id="value/>"code
+                                       />
+                                       <span>test</span>
+HTML
+                               ,
+                               'expected' => <<<HTML
+                                       <hr class="firstTag" foo="bar" id=">"code
+                                       <hr id="value>"code
+                                       <hr id="/>"code
+                                       <hr id="value/>"code
+                                       />
+                                       <span class="secondTag">test</span>
+HTML
+                       ,
+                       ),
+                       '<hr id   =5>'                                 => array(
+                               'input'    => '<hr id   =5><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" id   =5><span class="secondTag">test</span>',
+                       ),
+                       '<hr id a  =5>'                                => array(
+                               'input'    => '<hr id a  =5><span>test</span>',
+                               'expected' => '<hr class="firstTag" foo="bar" id a  =5><span class="secondTag">test</span>',
+                       ),
+               );
+       }
+}
</ins></span></pre>
</div>
</div>

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