Re-add <source media> for media elements

[zcorpan]

Fixes #6363

• At least two implementers are interested (and none opposed):
  • WebKit: already supported; positive
  • Gecko: positive
  • Chromium: positive
• Tests are written and can be reviewed and commented upon at:
  • HTML: Add <source media> to media elements web-platform-tests/wpt#40330
• Implementation bugs are filed:
  • Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=1450316
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1836128
  • WebKit: N/A
  • 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: <source media> in media elements is allowed again (spec change) mdn/content#27079

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

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/embedded-content.html ( diff )
/indices.html ( diff )
/media.html ( diff )

[foolip]

@dalecurtis what do you think about implementing this in Chromium?

This was once implemented in Chromium (inherited from WebKit) but I removed it in https://bugs.chromium.org/p/chromium/issues/detail?id=338197. In hindsight I think that was the wrong call, and I'm glad it's being reverted in the spec now.

[dalecurtis]

It seems reasonable and is something folks have been asking for. I'm not sure about implementation complexity, but the removed code doesn't look too large -- so it shouldn't be too bad.

I can only think of one minor issue at this time: It may interact in ways surprising to developers if an error occurs during loading. I.e., it could look like a resource wasn't selected due to the query but instead due to a load error that causes the next source child to be loaded.

[zcorpan]

@dalecurtis the same applies for type. We don't currently have a way to indicate why a resource wasn't loaded when using source elements (when using <video src> reason is available in video.error). It would be possible to expose with the error event (in the "Failed with elements" step), but this would be a separate issue.

[annevk]

Nice, seems like most of the editorial part of the work was already done due to picture.

[zcorpan]

@annevk thanks, nits fixed.

[zcorpan]

I've written tests and filed bugs.

[janwidmer]

@scottjehl any idea, when this feature lands in Firefox / Chrome Production?