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Re: Using preload for "async" CSS by changing `rel` trick

From: Ben Maurer <ben.maurer@gmail.com>
Date: Sat, 13 Jan 2018 10:39:55 -0800
Message-ID: <CABgOVaJz91JhnRSFp2r9eEE1NXXjCvKRR=Z1YGPBGxJpMROmzw@mail.gmail.com>
To: Ilya Grigorik <igrigorik@google.com>
Cc: Anne van Kesteren <annevk@annevk.nl>, Philip Walton <philipwalton@google.com>, Royi Hagigi <royi@fb.com>, "public-web-perf@w3.org" <public-web-perf@w3.org>
If you append a <style> to the head, the results apply synchronously:


However here's an example of a confusing behavior where I use a data uri on
a link tag.


in Safari, if I quit the browser the first time I load the page the text of
the element is rbg(255,0,0) [red]. If I reload the page the content is
rbg(0,0,255). This suggests that the parsing of the css is async but
there's a synchronous cache that can be used.


On Sat, Jan 13, 2018 at 10:23 AM, Ilya Grigorik <igrigorik@google.com>

> On Fri, Jan 12, 2018 at 11:43 PM, Anne van Kesteren <annevk@annevk.nl>
> wrote:
>> On Sat, Jan 13, 2018 at 7:39 AM, Ilya Grigorik <igrigorik@google.com>
>> wrote:
>> > On Fri, Jan 12, 2018 at 10:24 PM, Ben Maurer <ben.maurer@gmail.com>
>> wrote:
>> >>>> We use the fact that a resource has been downloaded to reveal content
>> >>>> that depends on the JS/CSS that we are fetching. So if rel=preload
>> told use
>> >>>> a.css is available we'll insert HTML that uses a.css into the DOM.
>> Even a
>> >>>> single frame of the browser doing something like parsing the file
>> would
>> >>>> create a disruptive user experience.
>> >>>
>> >>> Right, I see. I think that's a great use case to raise against HTML
>> spec
>> >>> and see what the guidance is there — this is not preload specific,
>> same
>> >>> applies for resources coming from HTTP cache, serviceworker, etc.
>> >>
>> >> Is the correct formulation of the question "does the presence of a
>> >> resource in the fetch group's response cache [which is what the
>> preload spec
>> >> says it adds the stylesheet to] guarantee that inserting a <link> with
>> an
>> >> href to that resource will synchronously apply to the DOM"
>> >
>> > I'll defer to Anne on this one. :-)
>> Well, you shouldn't, since this is the complaint I've been making to
>> you for a number of years now about the state of these features. You
>> haven't defined them end-to-end, so the answer to these questions
>> remains unclear.
> Anne, what's missing for preload specifically? Happy to address it if we
> can.
> My observation here is that you can remove preload out of this question
> and rephrase it as: "If I programmatically inject a <link rel=stylesheet>
> what execution guarantees, if any, are provided?"
>> Having said that, I don't think we'd want to give
>> synchronous guarantees, and especially not that the resource is
>> already parsed as CSS and has the relevant data structures created.
> Is that true in practice today? Ben, have you checked if there are any WPT
> tests for these behaviors?
> ig
>> --
>> https://annevankesteren.nl/
Received on Saturday, 13 January 2018 18:40:21 UTC

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