Add lazy-loading for <video poster>

[zcorpan]

Add a new posterloading attribute to the video element which controls lazy-loading of the poster image.

Fixes #6636.

• At least two implementers are interested (and none opposed):
  • WebKit: Lazy loading video element poster attribute #6636 (comment)
  • Chromium: Lazy loading video element poster attribute #6636 (comment)
  • Gecko: ?
• Tests are written and can be reviewed and commented upon at:
  • HTML: test <video posterloading=lazy> web-platform-tests/wpt#36712
• Implementation bugs are filed:
  • Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=1379294
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1797962
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=247184
  • 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: Add documentation for <video posterloading=lazy> mdn/content#21912
• - validator issue is filed: Support <video posterloading=lazy> validator/validator#1465

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

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/indices.html ( diff )
/media.html ( diff )
/urls-and-fetching.html ( diff )

[zcorpan]

Merge conflict fixed.

[annevk]

This largely looks correct to me, modulo some formatting concerns.

[zcorpan]

@annevk thanks, fixed.

[zcorpan]

Adding do not merge yet until we align on the approach (new attribute vs use an img element), see #6636 (comment)

[zcorpan]

Closing based on recent discussion (#6636 (comment) onwards).