[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):

 While looking around for the above comment I also tried to find some
 comparison data for page load speed with blocking vs. defer vs. async
 scripts. Unfortunately I couldn't. Ended up doing some quick testing with
 a 100KB HTML with five scripts ~10KB each. The defer and async scripts
 were loaded in the head, the blocking scripts were concatenated and loaded
 in the footer.

 The result was somewhat surprising: blocking vs. defer vs. async
 differences of page load (until  DOM ready) were less than 3%. Perhaps I
 didn't set the test up properly or testing on local serer is not good
 enough.

 Thinking it would be really good to have some performance comparison to
 justify this. Ideally testing would be on a remote server, with and
 without browser cache, with concatenated "blocking" scripts (as in
 production), and on HTTP/1.1 and HTTP/2.0 if possible.

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Ticket URL: <https://core.trac.wordpress.org/ticket/12009#comment:102>
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