Intent to prototype: CSS font-variant-emoji

[Jonathan Kew]

Summary:
Some Unicode characters are defined to have the potential to render in
either "text" style, using traditional monochrome symbols, or "emoji"
style, using full-color fonts of various types.

For characters that could be rendered in either form, the
font-variant-emoji property allows an author to specify which should be
preferred, which may affect which font the browser selects.

Bug:
* https://bugzilla.mozilla.org/show_bug.cgi?id=1461589

Specification:
* https://drafts.csswg.org/css-fonts/#font-variant-emoji-prop

Standards Body:
* W3C CSS working group

Platform coverage: All

Preference: layout.css.font-variant-emoji.enabled

DevTools bug: n/a

Other browsers:
* Not yet implemented in either Blink or WebKit.

web-platform-tests:
* Parsing and rendering tests to be added under WPT css/css-fonts/.

- Jonathan