Add 'Run video.rAF callbacks' to rendering steps

[tguilbert-google]

Adds an extra step to the 'update the rendering' portion of the event
loop processing model, which invokes the 'run the video animation frame
callbacks' algorithm, immediately before the 'run the animation frame
callbacks' algorithm.

In other words, this change describes how callbacks registered via
video.requestAnimationFrame() must be run immediately before callbacks
registered via window.requestionAnimationFrame().

The full definition of HTMLVideoElement.requestAnimationFrame() and
related terms and algorithms can be found here:
https://wicg.github.io/video-raf/

• At least two implementers are interested (and none opposed):
  • Chromium (+ Edgium) are interested
  • No response from Firefox yet
  • No official position from Safari. Webkit-dev discussion: first -- last
• Tests are written and can be reviewed and commented upon at:
  • https://wpt.fyi/results/video-raf
  • https://github.com/web-platform-tests/wpt/tree/master/video-raf
• Implementation bugs are filed:
  • Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=1012063
  • Firefox: …
  • Safari: …

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

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[tguilbert-google]

• The current official spec name is "HTMLVideoElement.requestAnimationFrame()", which is a method name. Should this be named something else, such as "Video AnimationFrameProvider Extension"?
• In the spec reference, I used WICG. Should this be W3C instead?

[domenic]

Hey @tguilbert-google,

I think we should not reference the separate spec at all. This is a modification to HTMLVideoElement, and as such when that spec graduates from incubation, it should land in the HTML Standard itself, and not become a separate document.

So I would suggest morphing this pull request into something which incorporates the normative contents of https://wicg.github.io/video-raf/ into the HTML Standard, in the appropriate locations.

[tguilbert-google]

Ok, thanks @domenic!

I will put a hold on doing that until there is an official position from Firefox or Safari, and the spec graduates from incubation.

[tguilbert-google]

A small update: Safari has implemented video.requestVideoFrameCallback, and @youennf and I discussed merging the WICG/video-rvfc spec (renamed from WICG/video-raf) into the HTML standard at TPAC.

I am closing this PR and will create a new one referencing this one for context. I intend on starting this work in October. Thanks!