Add loading-lazy-media: lazy loading for <video> and <audio> elements

[hjanuschka]

Summary

Adds a new loading-lazy-media feature for the loading attribute on <video> and <audio> elements, allowing deferred media loading until the element is near the viewport.

Context

• Requested in: Add support for video and audio element lazy-loading using loading #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
• Chromium CL: https://chromium-review.googlesource.com/c/chromium/src/+/7511253
• Chromium tracking bug: https://issues.chromium.org/issues/469111735
• 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/
• Also submitted to web-features: Add lazy-loading for <video> and <audio> feature web-platform-dx/web-features#3837

Support data

• Chrome 147+: n d #1 -- behind the LazyLoadVideoAndAudio experimental flag
• Firefox: n (no support yet, positive standards position)
• Safari/WebKit: n (no signal yet)
• All other browsers: n (no support)

Notes

• Set as child of loading-lazy-attr (the existing img/iframe lazy loading feature)
• shown: false as per contributing guidelines for new features
• Validation passes (npm run validate)

[scottjehl]

Nice! +1

[Fyrd]

Thanks!

[hjanuschka]

@Fyrd do you know when there ll be a release?
added the caniuse feature, but CI fails web-platform-dx/web-features#3837 (guess because its not released yet)