[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:  azaozz
     Type:  enhancement    |      Status:  reopened
 Priority:  normal         |   Milestone:  Future Release
Component:  Script Loader  |     Version:  4.6
 Severity:  normal         |  Resolution:
 Keywords:                 |     Focuses:
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Comment (by azaozz):

 Replying to [comment:47 westonruter]:
 > Retiring `load-scripts.php` I think can also/rather be accomplished via
 service workers: #36995.

 Right. This can be done in couple of different ways. The point is that it
 makes sense as part of refactoring how we load scripts in general.

 > It would make sense to me that if you register a script with an `async`
 flag ''and'' a non-empty `deps` that this should result in a
 `_doing_it_wrong()`.

 I agree we can add `async` when registering scripts that don't have any
 dependencies and can never be used as a dependency. This makes most sense
 for default core scripts that plugins can use.

 > Just to be clear, I'm talking about WP script dependencies generally not
 `load-scripts.php` specifically. I think there is good value in adding
 `async` and `defer` support to the existing `WP_Scripts` system so that we
 can take advantage of them now.

 Yes but `load-scripts.php` is an integral part of how we load scripts.
 Using `async` and `defer` will have to pull a script out of there (prevent
 concatenation) and break the execution order. We can't do that for any of
 the default scripts, see the previous comment.

 > Since you noted that less than 25% of the Web is on HTTP/2, I don't
 think we should let it be the blocker for us to encourage best practices
 now for script loading for themes and plugins via `async` and `defer`.
 HTTP/2 definitely won't be available before Gutenberg.

 Yep, I realise that. Refactoring how we load scripts will slow down 3/4 of
 the sites currently. However looking at the speed HTTP/2 is being
 implemented, that number will be quite lower in six months, and possibly
 be less than 50% by the end of the year.

 I also agree with @mor10 that implementing it will give another push to
 faster switching to HTTP/2 everywhere.

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Ticket URL: <https://core.trac.wordpress.org/ticket/12009#comment:50>
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