[css-fonts] new generic font 'emoji'

Currently CSS defines a set of five generic fonts [1]:

  serif, sans-serif, monospace, cursive, fantasy

I'd like to propose expanding this to include a generic font for
emoji characters:

  emoji

The 'emoji' value would map to whatever emoji font is available on a
given platform (e.g. Apple Color Emoji, Segoe UI Emoji, Noto Color
Emoji).

Part of the motivation here is that when Unicode defined a mapping
of emoji characters into Unicode, it explicitly unified some of
these with existing symbol codepoints. If an author relies on system
font fallback to choose a font there's no guarantee an emoji font
will be prioritized over a symbol font, since it's difficult for a
user agent to distinguish between symbol vs. emoji usage. Specifying
the 'emoji' value in a fontlist would prioritize the use of color
glyphs for all codepoints covered by the emoji font.

Thoughts?

Cheers,

John Daggett
Mozilla Japan

[1] http://www.w3.org/TR/css3-fonts/#generic-font-families