[wp-trac] [WordPress Trac] #12009: Add support for HTML 5 "async" and "defer" attributes

[WordPress Trac]

#12009: Add support for HTML 5 "async" and "defer" attributes
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 Reporter:  Otto42                               |       Owner:  10upsimon
     Type:  enhancement                          |      Status:  assigned
 Priority:  high                                 |   Milestone:  6.3
Component:  Script Loader                        |     Version:  4.6
 Severity:  normal                               |  Resolution:
 Keywords:  has-patch has-unit-tests 2nd-        |     Focuses:
  opinion                                        |  performance
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Comment (by azaozz):

 Replying to [comment:126 westonruter]:

 > I’m confused because previously
 [https://core.trac.wordpress.org/ticket/12009?replyto=125#comment:86 you
 said] that "support for concatenation would not be a priority and probably
 won't be necessary.

 Yea, sorry, I didn't explain that well at all. The reasons I think support
 for concatenation is not needed in the first iteration (this patch) is
 because only default core scripts can be concatenated and only in wp-
 admin. As there are no default scripts that are (can be) loaded with
 `async` or `defer`, adding concatenation support seems an overkill. If
 eventually there are enough (new) core scripts that are loaded with
 `async` or `defer` by default, it seems concatenation can be added as that
 dramatically speeds up page load times on HTTP/1.2.

 > Actually, being able to transition applicable core scripts to use a
 delayed strategy is specifically what we want to explore once this gets
 merged (e.g. `comment-reply`).

 Yep, seems the only core script that may be able to be loaded with a
 delayed strategy is `comment-reply` (front-end) as it is self-contained
 and afaik not used as a dependency. However pretty much all of the rest of
 the core scripts are not self-contained and often used as dependencies by
 various plugins. In some cases some core scripts are also used in inline
 scripts, even scripts that are pasted in post_content. That makes it
 virtually impossible to "convert" them to a delayed loading without
 bringing regressions and edge cases.

 > We’ve specifically developed our implementation in a way that retains
 compatibility between core delayed scripts and theme/plugin blocking
 scripts. In particular, if a core script adopts a delayed strategy but a
 theme/plugin is enqueueing a blocking script that depends on the core-
 delayed script, then our implementation forces the core script to revert
 to being blocking. So we have implemented a back-compat method to maintain
 the loading and execution order, including for before/after inline
 scripts.

 That would work as long as a script file is enqueued through script-
 loader, but will usually fail for stand-alone inline scripts. There are
 even scripts that are pasted by admin/editor users in posts, etc. Seen
 this type of regressions before, even on big/modern sites...

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Ticket URL: <https://core.trac.wordpress.org/ticket/12009#comment:127>
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