Intent to prototype: CSS Anchor Positioning

[Jonathan Watt]

Summary: Support for positioned elements sizing and positioning themselves
relative to one or more "anchor elements" elsewhere on the page.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1838746

Specification: https://drafts.csswg.org/css-anchor-position-1/

Platform coverage: All

Preference: TBD (likely multiple)

DevTools bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1895141

Standards-positions: https://github.com/mozilla/standards-positions/issues/794

Blink:
In developer trial 124 (behind flag).
https://groups.google.com/a/chromium.org/g/blink-dev/c/jGTYNuidPRs
https://chromestatus.com/feature/5124922471874560

WebKit:
No official position and no bugs.webkit.org bug yet, but Apple co-edits the spec.
https://github.com/WebKit/standards-positions/issues/167#issuecomment-1708871010

web-platform-tests:
https://github.com/web-platform-tests/wpt/tree/master/css/css-anchor-position
https://searchfox.org/mozilla-central/source/testing/web-platform/tests/css/css-anchor-position
https://wpt.fyi/results/css/css-anchor-position