Use width and height as intrinsic aspect ratio for img elements

[emilio]

When the current request is not available, or it doesn't have an intrinsic
ratio (such as some SVG content).

This specifies the proposal in
WICG/intrinsicsize-attribute#16, which is implemented
in both Firefox and Chromium behind a flag.

Tests for this change are at:

https://github.com/web-platform-tests/wpt/blob/a57ec1432f22ac42e8e219a32e2abd7c0baa5b09/html/rendering/replaced-elements/attributes-for-embedded-content-and-images/img-aspect-ratio.tentative.html

• At least two implementers are interested (and none opposed):
  • Firefox (implements behind the flag layout.css.width-and-height-map-to-aspect-ratio.enabled).
  • Chromium (behind the AspectRatioFromWidthAndHeight flag).
• Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/blob/a57ec1432f22ac42e8e219a32e2abd7c0baa5b09/html/rendering/replaced-elements/attributes-for-embedded-content-and-images/img-aspect-ratio.tentative.html
• Implementation bugs are filed:
  • Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=979891
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1547231
  • Safari: https://bugs.webkit.org/show_bug.cgi?id=201641

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

---

/infrastructure.html ( diff )
/rendering.html ( diff )

[annevk]

Thanks I like this second take modulo a couple nits.

[annevk]

This looks good to me, modulo some formatting nits I can push a fixup for. @domenic do you want to review this? Anyone else?

[domenic]

I'm on vacation so happy to leave this to you and/or others.

[annevk]

@fantasai @cbiesinger any thoughts on this? If not I plan to land this Wednesday.

[fantasai]

Looks reasonable to me. I'd link to https://www.w3.org/TR/css-images-3/ instead, but, uh, I forgot to submit the publication request last month so it's not up yet. ^_^;;;;; Will be fixed hopefully sometime next week.. w3c/transitions#170

[domenic]

We dont, as a rule, link to /TR/ documents from WHATWG specs, so no rush there :).

[fantasai]

Seems to only apply to IMG, but should probably also apply to VIDEO?

[cbiesinger]

Looks good to me. We may want to include video but neither Blink nor Gecko support that right now.

[emilio]

Seems to only apply to IMG, but should probably also apply to VIDEO?

Yeah, we should probably add <video> too, and maybe other things, like <object> / <embed> / <input type="image"> / etc...?

Hoewever <img> is the most useful (as in broadly used), and the easiest to implement interoperably.

Also <video> has the extra trickiness of the poster image affecting its intrinsic size (which is just weird), and <img> has the "current" vs. "pending" request stuff, so it'd probably be quite different spec text anyway.

So yeah I'd rather land and ship <img>, then also implement and spec <video> (I'll file a follow-up on this repo to do that).

[annevk]

Filed #4961 as follow-up.

[fantasai]

@emilio input type=image certainly, but embed/object are also used for things which do not generally have an intrinsic aspect ratio, so I wouldn't apply it there.