Intent to Prototype: CSS anchor positioning with transforms

[Ting-Yu Lin 林庭宇]

Summary:
When an anchor element is transformed, the anchor-positioned element's position and size are resolved against the bounding rectangle of the anchor's transformed border box, rather than the untransformed one. This keeps anchor-positioned UI such as tooltips attached to their anchor while the anchor is being transformed.

This is a behavior change to anchor positioning, which already ships. Authors will be able detect the feature with @supports named-feature(anchor-position-follows-transforms).

Bug:
[meta] https://bugzilla.mozilla.org/show_bug.cgi?id=1993692

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

Standards Body:
CSS WG

Platform Coverage:
All

Preference:
layout.css.anchor-positioning.follows-transforms.enabled

DevTools Bug:
N/A

Extensions Bug:
N/A

Use Counter:
N/A (Extension to already shipped feature)

Standards-Positions Discussion:
https://github.com/mozilla/standards-positions/issues/1302

Other Browsers:
    Blink: Shipped https://groups.google.com/a/chromium.org/g/blink-dev/c/48-VhjHY3Lc/m/HkC8aHMnAQAJ
    WebKit: Prototyping https://github.com/WebKit/standards-positions/issues/558

web-platform-tests:
https://wpt.fyi/results/css/css-anchor-position/ (specifically the transform-*.html tests)