[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:40 mor10]:
 > Concatenating JavaScript is an old patch to get around single-stream
 transfer under HTTP/1.1. With HTTP/2 multiplexing...

 All of this sounds pretty good, in theory. I wish we could do that right
 now. The problem is that less than 25% of the websites currently support
 HTTP/2: https://w3techs.com/technologies/details/ce-http2/all/all.

 > > In addition `async` cannot be used if the script has dependencies or
 is used as dependency for another script.

 > Not entirely accurate.

 The `async` attribute cannot be used in script-loader because the script
 doesn't follow execution order. If you are adding a stand-alone script
 that has no dependencies and other scripts don't depend on it, there is no
 point adding it to script loader at all.

 Right now plugins and themes can load all scripts in the head and add
 `defer` if they really want by using the `'script_loader_tag'` filter.
 This will make 75% of sites slower. Whether we should make this easier is
 another question. I'd rather we don't.

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Ticket URL: <https://core.trac.wordpress.org/ticket/12009#comment:43>
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