Add controls attribute to img

[yoelhawa]

Add a new controls attribute to img that allows the user agent to add interactive user interface over the image.

Fixes #11856.

• At least two implementers are interested (and none opposed):
  • WebKit
  • Chromium: neutral
  • Gecko: neutral
• Tests are written and can be reviewed and commented upon at:
  • Will be covered by IDL tests.
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/484367301
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2016875
• Corresponding HTML AAM & ARIA in HTML issues & PRs: New controls attribute for img element w3c/html-aam#601
• MDN issue is filed: Add <img controls> mdn/content#43730
• The top of this comment includes a clear commit message to use.

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

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[annevk]

Looks good, nice! You also need to update img in the element index.

[annevk]

@yoelhawa can you update this PR so there's an open issue with a pointer to #12318 as suggested by @zcorpan? You want to use class="XXX" and probably emulate one of the existing ones that points to a GitHub issue. The general gist of the wording should be that we don't want user agents to add animation controls before tackling that issue. Thanks!

[zcorpan]

There was discussion in w3c/html-aam#601 about how this impacts the a11y exposure. It seems solvable to me, but should it block landing this? What's the a11y mapping in WebKit?

[yoelhawa]

Left a comment there about WebKit's a11y implementation. I agree that this seems easily solvable.