[wp-trac] [WordPress Trac] #53070: Establish a Core CSS deprecation path

[WordPress Trac]

#53070: Establish a Core CSS deprecation path
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 Reporter:  isabel_brison     |      Owner:  (none)
     Type:  enhancement       |     Status:  new
 Priority:  normal            |  Milestone:  Awaiting Review
Component:  General           |    Version:
 Severity:  normal            |   Keywords:
  Focuses:  css, performance  |
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 Problem: Core CSS is a public API, and gets used as such by plugins. This
 means that when a selector is no longer used in wp-admin, deleting any
 related CSS rules can still break third party code. However, the cautious
 approach of not deleting any CSS leads to bloated files, with a negative
 impact on performance.

 Solution: define a clear deprecation path for Core CSS, announcing in
 advance when any selector and related rules are being deprecated. This
 allows third parties to update their code in a timely manner.

 What will this look like in practice? As discussed in the CSS meeting:
 [https://make.wordpress.org/core/2021/04/13/css-chat-
 summary-08-april-2021/],

 * When a selector is no longer used in Core, move related CSS to a
 `deprecated.css` file.
 * This file can be optionally enqueued by plugins that require any of its
 CSS.
 * Deprecation is announced in the release dev notes.
 * Linters and other automated tools can help enforce non-use of deprecated
 selectors.

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Ticket URL: <https://core.trac.wordpress.org/ticket/53070>
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