Intent to Prototype and Ship: named-feature() function for CSS @supports

[Ting-Yu Lin 林庭宇]

Summary:
named-feature() is an @supports function that lets authors detect features that cannot be expressed with any other @supports mechanism.

One of the features Gecko is developing is CSS anchor positioning with transforms [1]. Once it is ready, authors can detect the feature with @supports named-feature(anchor-position-follows-transforms). 

Bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=2042977
https://bugzilla.mozilla.org/show_bug.cgi?id=2055354

Specification:
https://drafts.csswg.org/css-conditional-5/#support-definition-named-features
https://drafts.csswg.org/css-conditional-5/#typedef-supports-named-feature-fn

Standards Body:
CSS WG

Platform Coverage:
All

Preference:
This is a simple feature requiring no dedicated preference. I'm planning to ship it in Firefox 156. That is, the detection mechanism is in place, but every named-feature() query evaluates to false at the moment.

Once layout.css.anchor-positioning.follows-transforms.enabled is enabled, named-feature(anchor-position-follows-transforms) will evaluate to true.

DevTools Bug:
N/A

Extensions Bug:
N/A

Use Counter:
N/A

Standards-Positions Discussion:
https://github.com/mozilla/standards-positions/issues/1340

Other Browsers:
    Blink: Shipped https://groups.google.com/a/chromium.org/g/blink-dev/c/jRufi9adKY0/m/0xJTluPPBgAJ
    WebKit: Prototyping https://github.com/WebKit/standards-positions/issues/600

web-platform-tests:

https://wpt.fyi/results/css/css-conditional/at-supports-named-feature-001.html

[1] "Intent to Prototype: CSS anchor positioning with transforms." https://groups.google.com/a/mozilla.org/g/dev-platform/c/ESPZgAnSDss/m/0cf-kcITEAAJ