Intent to ship: CSS '-webkit-fill-available' sizing keyword in the 'height' and 'width' properties

[Daniel Holbert]

As of today (2025-09-17), I intend to turn on support for the CSS '-webkit-fill-available' sizing keyword, specifically in the 'height' and 'width' properties. (Support for the keyword in other properties will ship soon but separately, per note at the bottom of this message.)

It has been developed behind the 'layout.css.webkit-fill-available.enabled' preference. Other WebKit/Blink-based browsers have already been shipping this legacy prefixed keyword for over a decade.

Bug to turn on by default: https://bugzilla.mozilla.org/show_bug.cgi?id=1988938

Standard: https://drafts.csswg.org/css-sizing-4/#stretch-fit-sizing

(Note on the standard/spec: this webkit-prefixed keyword is a legacy alias for what is now standardized as 'stretch' in that spec.  Gecko and WebKit have not yet shipped that standardized 'stretch' keyword, but there's strong webcompat dependence on this legacy webkit-prefixed keyword keyword, particularly in the 'width' and 'height' properties - hence the need to ship that version as soon as possible.)

This feature was previously discussed in this "Intent to prototype" thread:

https://groups.google.com/a/mozilla.org/g/dev-platform/c/-pMSV-kgUjA/m/VRL09R1NAAAJ

Note: In the coming weeks, I also intend to enable support for this prefixed keyword in other properties (beyond 'height' and 'width') as well - that support is guarded by the "layout.css.webkit-fill-available.all-size-properties.enabled" preference (added in https://bugzilla.mozilla.org/show_bug.cgi?id=1988301 ).  And I intend to enable support for the standardized keyword name ('stretch') by the end of the year.  I'll send separate intent-to-ship messages for those changes.