Intent to prototype & ship: CSS font-palette property and @font-palette-values rule

[Jonathan Kew]

Summary:
Color fonts (such as emoji, icons, or decorative alphabets) may include
multiple color palettes, e.g. with colors specifically chosen to work
well on a light or a dark background. The CSS font-palette property
allows an author to select which palette should be used.

The @font-palette-values rule allows an author to customize the color
palette, so that (for example) a collection of decorative initial
letters could be modified to adopt the colors of a site's theme.

Bug:
* https://bugzilla.mozilla.org/show_bug.cgi?id=1461588

Specification:
* https://drafts.csswg.org/css-fonts/#font-palette-prop
* https://drafts.csswg.org/css-fonts/#font-palette-values

Standards Body:
* W3C CSS working group

Platform coverage: All

Preference: layout.css.font-palette.enabled

DevTools bug: n/a

Other browsers:
* Supported in both WebKit and Blink.

web-platform-tests:
*
https://wpt.fyi/results/?label=master&label=experimental&aligned&view=subtest&q=font-palette


The implementation is landing in Nightly this week, and then I propose
to enable the pref by default in the 107 cycle, so that these color-font
authoring features will ship at the same time as support for the new
COLRv1 enhanced color font format (see
https://groups.google.com/a/mozilla.org/g/dev-platform/c/Xs0_dixyT9Q).

Bug to turn on by default:
* https://bugzilla.mozilla.org/show_bug.cgi?id=1794091

- Jonathan