Add lazy-loading for <video> and <audio> feature

[hjanuschka]

Summary

Adds a new loading-lazy-media feature for the loading="lazy" attribute on <video> and <audio> elements.

This extends the existing lazy-loading behavior (already available for <img> and <iframe> via the loading-lazy feature) to media elements.

Context

• Requested in: New feature: Lazy loading for video and audio elements #3719
• Also tracked in caniuse: Add support for video and audio element lazy-loading using loading Fyrd/caniuse#7467
• WHATWG HTML spec PR: Add support for video and audio element lazy-loading via the loading attribute whatwg/html#11980
• Chromium intent to ship: https://groups.google.com/a/chromium.org/g/blink-dev/c/DNVOVe4VNak/m/rCcnWz8QAQAJ
• Mozilla standards position (positive): Add support for video element lazy-loading via the loading attribute mozilla/standards-positions#1325
• Blog post by Scott Jehl: https://scottjehl.com/posts/lazy-media/

Feature details

• ID: loading-lazy-media
• Name: Lazy-loading media elements
• Spec: https://html.spec.whatwg.org/multipage/urls-and-fetching.html#lazy-loading-attributes
• Group: media-elements
• Status: baseline: false (no browser ships this yet)
• compat_features: omitted -- no BCD keys exist yet. Keys like html.elements.video.loading, html.elements.audio.loading, api.HTMLVideoElement.loading, and api.HTMLAudioElement.loading should be added once they land in BCD.

[ddbeck]

Thanks for opening this, @hjanuschka. Some suggestions in line comments.

Also, if Fyrd/caniuse#7488 merges first, then you must add caniuse: loading-lazy-media.

If this PR merges first, then I would recommend closing the caniuse PR.

[hjanuschka]

all done :)

[ddbeck]

OK, cool. I'm going to merge main into this branch, which ought to make the tests pass.

[ddbeck]

More a note for any other maintainers watching: so now we have a situation where caniuse has beaten us to the punch and we've got a different name. I like that our name matches the rest of the feature set, so I'm not going to suggest changing it here, but it would have been nice to align with Alexis. 😞 I've put this on the agenda for he and I to talk about it on our next call.