W3C home > Mailing lists > Public > public-respimg@w3.org > February 2013

Re: CfC: to publish "The srcset attribute" specification as a First Public Working Draft (FPWD)

From: Nathanael D. Jones <nathanael.jones@gmail.com>
Date: Wed, 6 Feb 2013 10:33:10 +0100
Message-ID: <CAG3DbfVz++OK8RSD+Dw1iHhO34KV7t1WHBjK+Uif-TBZGC+=hQ@mail.gmail.com>
To: Yoav Weiss <yoav@yoav.ws>
Cc: Mat Marquis <mat@matmarquis.com>, "public-respimg@w3.org" <public-respimg@w3.org>

Prefetching does *not* need to be delayed until layout is calculated. I'm
suggesting that prefetching be performed the same way it is currently done
for CSS background-images. Once the markup and the css have been
cross-referenced, the download begins. This can occur as early as the first
CSS signal tag (if spec'd), the first non-media query selector after a
matching rule, or as late as the end of the last CSS request. From what I
understand about CSS parser implementations, an incremental data structure
is available during parsing, which should offer enough data to make an
informed pre-fetch decision.

If javascript later creates a new element or changes key parameters (such
as min-width/min-height,max-width/max-height), then a final fetch is
required. This is the status quo for background-image and img prefetching,
so why the fixation on waiting until render for *pre*fetching?

Pre-fetching, by nature, is optimistic. We don't wait until render to
pre-fetch CSS background-images, despite them being more likely to be
affected; why assume a worst-case browser implementation for `<picture>`?


I communicate with customers implementing responsive image resizing (and
art direction based on face detection) on a daily basis; it's my job, and
RESTful image processing is my company's focus. We already have an
implementation based on the current <picture> spec, and frankly, the harder
`<picture>` is to use directly, the more people will need our help.... but
that's not what's best for the web.

1. I'm not suggesting *any* changes to CSS Media Queries whatsoever
2. I'm not suggesting we consult the size of the container, rather the CSS
size *constraints* (min-*, max-*) of the picture element itself. Unless
percentage units are used, informed *pre*fetching can occur before layout.
3. The naming of height/width attributes is unimportant. I would not
consider it repurposing, as they would behave as if they were applied to a
'img' element, but if it harms backwards compatibility somehow, let's look
at alternate options, such as w/h and size.
4. Thus my e-mail to you on the 4th, in hopes you would provide guidance on
the appropriate forum for this discussion.

Best regards,
Nathanael Jones

On Tue, Feb 5, 2013 at 11:53 PM, Yoav Weiss <yoav@yoav.ws> wrote:

> Nathanael,
> I'd like to add that you are highly underestimating the amount of delay
> postponing fetching of images until after the page has layout will
> introduce.
> The average delay of a 4G network is around 150 ms. For a 3G network it is
> around 400 ms [1]
> That means that in the best case scenario of a page with a single small
> CSS (that fits into the initial congestion window), and no External
> JavaScript in the head, we are looking at at least 400 ms delay in the
> page's overall page load for 3G users. That is a lot!
> Let's look at a real life loading scenario of a Web page containing
> several CSS files from different hosts (external font library is a common
> case), as well as a few external JavaScript files from various hosts (CDN
> based JQuery & local scripts)
> In this case you are looking at at least 2-3 delays more (establishing
> connections, requesting & fetching resources, which at least some are
> bigger than the 15KB of the initial congestion window), even with SPDY.
> Add to that CSS & JS parsing & rendering/execution, creation of DOM,
> RenderTree and Layout (none of which is negligable on mobile devices), and
> you have added 2-3 seconds to an average page.
> I'm afraid that the performance regression that will result from
> waiting for layout before requesting images is just too much for it to be
> a viable option.
> Yoav
> [1]
> http://www.igvita.com/2012/07/19/latency-the-new-web-performance-bottleneck/
> On Tue, Feb 5, 2013 at 11:27 PM, Mat Marquis <mat@matmarquis.com> wrote:
>> On Feb 5, at 3:36 PM, Nathanael D. Jones wrote:
>> I think many people were expecting a solution based on element size.
>> I would be interested in seeing data to support this assertion.
>> While that may not be a use-case you've aimed to solve, *I think it
>> should be the *primary* use-case*, as it provides a *simple solution to
>> nearly every other use-case documented*.
>> *Here's my full proposal:*
>> https://gist.github.com/nathanaeljones/4706093
>> Where your goal is to selectively load images based on the size of the
>> containing element and you’re comfortable delaying the requests for those
>> assets until after the layout has been painted, the functionality you’re
>> looking for could be easily accomplished using JavaScript and data
>> attributes.
>> In terms of the above as a native solution: your proposal involves
>> heavily modifying the syntax of media queries (with syntactical overlap
>> between your proposal and the existing method of specifying media types),
>> repurposing `width` and `height` attributes, and delaying image requests to
>> well beyond the initial parsing of the markup (waiting until CSS and JS is
>> requested, transferred, and rendered to begin requesting image sources). As
>> a first step, I might recommend reaching out to browser representatives to
>> determine the implementation viability of your proposal—there are several
>> members of the RICG that would likely be willing to offer you feedback.
>> I understand that this is a subject to which a great deal of people have
>> given thought. I’m happy to continue this discussion on the RICG mailing
>> list, but I’m sure you can understand where we won’t be withdrawing the
>> `picture` proposal based on proposals that haven’t yet been thoroughly
>> vetted or discussed at any length. The HTML WG Administrative list doesn’t
>> seem like the appropriate venue for those discussions.
>> Thanks,
>> Mat Marquis
>> For the record:
>> I do not believe the advantages of slightly-earlier prefetching outweigh
>> the benefits of a CSS-based approach. There are many possible optimizations
>> available to ensure the delay can be reduced to ~40ms for a cache miss
>> (Probably ~15ms with SPDY), and it is simply not worth the markup
>> complexity required.
>> Best regards,
>> Nathanael Jones
>>  [ snip ]
Received on Wednesday, 6 February 2013 09:34:00 UTC

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