Deal with content-visibility during the layout loop.

[emilio]

• At least two implementers are interested (and none opposed):
  • Gecko
  • …
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • 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
• Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A
• MDN issue is filed: N/A, I believe.
• The top of this comment includes a clear commit message to use.

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

[emilio]

Turned this into a draft because I think this needs:

• [css-contain-2] WIP: Stop monkey-patching the HTML spec. w3c/csswg-drafts#12105 to build.
• Some discussion about the details in CSS content-visibility content relevancy section should be in the HTML spec. #11210.
• Tests (that I'll write once we're clear on the details).