[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:114 westonruter]:
 > There are other cases where async scripts occur in plugins which also
 involve after scripts, but they are manually printed. For example (links
 are to source code):

 The consideration here, imho, is that:
 1. Adding "async" attribute to script tags is not a new feature. It has
 existed for many years, and many WP plugins (3000+) are already using it.
 2. Estimate for the "conversion ratio" if/when support for the "async"
 attribute is added to the Script Loader: My guess is that only a small
 number of plugins that currently add "async" will switch to using Script
 Loader. Generally only plugins that want to let third-party code (other
 plugins) to remove their scripts would have a reason to switch. All the
 rest will not have a reason. In these terms my estimate is that at most
 about 5% of the plugins will switch to Script Loader which would be
 unsatisfactory/poor conversion.
 3. It seems reasonable to expect the plugins to handle cases of the rarely
 used "script after" the same way plugins are expected to handle cases for
 the "undefined" load order for "async" scripts. I.e. plugins that
 absolutely must use a script that runs immediately after an "async" script
 finishes running, and cannot do that from JS, should hande that
 themselves, same as currently.

 Note that there are better alternatives to run an inline script after a
 script was run:
 1. Can be implemented in the main script. This is by far the best option.
 2. The after-script can be run on the `DOMContentLoaded` event. This event
 fires after all scripts, blocking, defer, and async have been loaded and
 executed.

 In these terms I'd consider the Script Loader support for "after" scripts
 for scripts with async and defer as **not-a-good idea**. The alternatives
 are better.

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Ticket URL: <https://core.trac.wordpress.org/ticket/12009#comment:117>
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