[wp-trac] [WordPress Trac] #59316: Incorporate script loading strategies in bundled themes

[WordPress Trac]

#59316: Incorporate script loading strategies in bundled themes
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 Reporter:  westonruter              |      Owner:  (none)
     Type:  enhancement              |     Status:  new
 Priority:  normal                   |  Milestone:  6.4
Component:  Bundled Theme            |    Version:  6.3
 Severity:  normal                   |   Keywords:
  Focuses:  javascript, performance  |
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 As of #12009 the script loader now has the ability for scripts to be
 printed with `async` or `defer`. This is being used on the frontend now
 for block view scripts (#59115), the `comment-reply` script (#58870), and
 the `wp-embed` script (#58931). Nevertheless, these new script loading
 strategies are not currently being used in the bundled themes. There are
 opportunities to improve performance by doing so.

 It turns out that Twenty Twenty actually has a custom implementation of
 script loading strategy injection which can now be deprecated in favor of
 what is in core.

 In general, any script that involves anything to do with the header
 (initial viewport) should get moved to the `head` (if it was registered as
 being `$in_footer`) and applied the `defer` loading strategy. This will
 ensure that the script will start loading earlier so that as soon as the
 DOM has loaded the logic can run and minimize the amount of time that the
 initial viewport is in an uninitialized state. Note that `defer` is
 preferable to `async` for scripts in the `head` because if the script is
 cached an `async` script can actually block rendering whereas a `defer`
 script never will.

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Ticket URL: <https://core.trac.wordpress.org/ticket/59316>
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