Intent to Ship: alpha() relative color function

[Sajid Anwar]

As of Firefox 155, I intend to enable the CSS alpha() relative color function on all platforms. This feature was developed behind the layout.css.alpha-color-function.enabled preference. Implementations are in progress in both Blink and WebKit.

Note that since the initial prototype implementation, the CSSWG has made multiple relevant updates to the specification for alpha():

• w3c/csswg-drafts#13994
  • Adds missing serialization details for the resolved value of the alpha() relative color function. This requires that they serialize using the modern color() syntax.
• w3c/csswg-drafts#14070
  • Makes the alpha value required to be specified in order for the function to be valid, as it is the only controllable component. This disallows values like alpha(from red).
  • Updates the serialization of the declared value of all relative color functions to always serialize the alpha value (if specified).

Firefox's implementation that will ship supports these edits/resolutions. WPT test cases for these edits have been merged (Bug 2059737) or are in progress (Bug 2059988).

Bugs:
- Bug 2042646 - Implement alpha() function - Main bug.
- Bug 2059738 - Set layout.css.alpha-color-function.enabled for all users - Bug to launch feature.

Standard:
https://drafts.csswg.org/css-color-5/#relative-alpha

Intent to Prototype Thread:
https://groups.google.com/a/mozilla.org/g/dev-platform/c/3jC_H0oP5f8