Force size containment for auto-sizes

[zcorpan]

The 'concrete object size ignoring natural dimensions' concept was not implementable. This change makes sizes=auto required on img to use auto-sizes (still optional on source) and forces size containment so that the image size is the same before and after an image has loaded, to prevent double downloads.

Fixes #9448.

• At least two implementers are interested (and none opposed):
  • Chromium
  • Gecko
• Tests are written and can be reviewed and commented upon at:
  • HTML: test UA styles for <img sizes=auto> web-platform-tests/wpt#41817
  • https://chromium-review.googlesource.com/c/chromium/src/+/5064461
• Implementation bugs are filed:
  • Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=1359051
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1819581
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=253143
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): N/A
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): N/A
• MDN issue is filed: Document img auto-sizes mdn/content#30596

(See WHATWG Working Mode: Changes for more details.)

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/embedded-content.html ( diff )
/images.html ( diff )
/rendering.html ( diff )

[joemcgill]

This may be somewhat off-topic (please feel free to redirect me, if so) but one aspect of the parsing algorithm is still unclear to me. If a platform (e.g. WordPress) generated image markup that looks like this (note the omission of a loading="lazy" attribute):

<img src="200.jpg" width="200" height="200" srcset="200.jpg 200w, 400.jpg 400w, 600.jpg 600w" sizes="auto, 200px" />

Would the omission of the loading attribute cause sizes to be calculates as the default 100vw value, would the 200px value be respected, or would size containment come into play? I would hope that the the "200px" value would be respected in this case so that platforms could add auto to the beginning of a sizes and dynamically decide whether to add a loading attribute while constructing the markup. In other words, if auto can't be applied because the image is not being lazy loaded, then the browser should respect the next items in the media condition.

[zcorpan]

@joemcgill right now, the intent is that auto would be returned by the "parse a sizes attribute" algorithm, and evaluate to 100vw. But I can see that falling back to the next value can be useful. I'll file a new issue.

[zcorpan]

I wrote tests for the UA stylesheet in web-platform-tests/wpt#41817

I pushed 1b6f84a to fix #9648 and #9649. Still need to write tests for the sizes parser changes.

[foolip]

@zcorpan any idea why PR preview is failing here? What's the best way to retry it?

[foolip]

I've done an editorial review and tried to ensure this PR does what it claims, but I have not gone through the history of #9448 to understand this deeply. I trust that prospective implementers will evaluate the implementability when reviewing this.

[tcaptan-cr]

LGTM

[zcorpan]

@tcaptan-cr has implemented this in Chromium and created remaining tests, see https://bugs.chromium.org/p/chromium/issues/detail?id=1359051

I believe this is ready to merge!

[foolip]

I've reviewed the changes since #9493 (review)