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Re: Clarification on `async` and `defer` together

From: Stu Cox <stuart.cox@gmail.com>
Date: Mon, 8 Jul 2013 13:16:56 +0100
Message-ID: <CAJ-2Ov7jnZf7GbQAsXNfb=T7qebkwzzFLAJR=W=nEt94=cjr9A@mail.gmail.com>
To: Jake Archibald <jaffathecake@gmail.com>
Cc: www-html@w3.org
On 7 July 2013 17:05, Jake Archibald <jaffathecake@gmail.com> wrote:

> From http://www.w3.org/TR/html5/scripting-1.html#script "The defer
> attribute may be specified even if the async attribute is specified, to
> cause legacy Web browsers that only support defer (and not async) to fall
> back to the defer behavior"
> In step 15 of http://www.w3.org/TR/html5/scripting-1.html#prepare-a-scriptthe defer behaviour is only applied "If the element has a src attribute,
> and the element has a defer attribute, and the element has been flagged as
> "parser-inserted", and the element does not have an async attribute", so
> the defer behaviour doesn't apply if the async attribute is present.
> Cheers,
> Jake
> On 7 July 2013 14:59, Stu Cox <stuart.cox@gmail.com> wrote:
>> Not entirely sure if this is the right reference doc:
>> http://www.w3.org/TR/html-markup/script.html  but it doesn't make it
>> clear what should happen if both `async` and `defer` attributes are set on
>> a `<script>` element:
>> <script src="myscript.js" async defer>
>> Browsers all seem to give `async` precedence, and I agree that this is
>> the most desirable behaviour, however logically `async` and `defer` aren't
>> necessarily mutually exclusive. If both were applied, it might be
>> reasonable to assume the behaviour might match that of `defer` alone.
>> Could a note be added to this / any other relevant docs highlighting this
>> behaviour?
>> The WHATWG spec [1] suggests this behaviour, but isn't incredibly clear
>> about it either.
>> [1]
>> http://www.whatwg.org/specs/web-apps/current-work/multipage/scripting-1.html
Ok, that's clear, thanks Jake.

Maybe all that's missing is a similar comment on that working group note
then: http://www.w3.org/TR/html-markup/script.html  it's the first Google
search result for "w3c script async" and some other related searches.

Stu Cox
Received on Monday, 8 July 2013 12:17:55 UTC

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