Add support for video element lazy-loading via the loading attribute

[scottjehl]

Specify support for the loading attribute for video using similar conventions to img and iframe, where the attribute has precedent. It has possible values of eager and lazy. When the loading attribute value is lazy, loading of any video or poster image data, as well as autoplay playback, must defer until layout is known and the video is in the viewport. This commit contains examples for how the loading attribute should take precedence over the preload, poster, and autoplay attributes. These were informed by experiments with local patch implementations to several user agents.

Addresses #10376

• At least two implementers are interested (and none opposed):
  • WebKit Pending Position: Add support for video element lazy-loading via the loading attribute WebKit/standards-positions#586
  • Gecko Pending Position: Add support for video element lazy-loading via the loading attribute mozilla/standards-positions#1325
  • Chromium: ...
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): …
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): …
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed: …
• 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]

cc @whatwg/media