[css-color] Consider exposing native accent color as a system color keyword. #7347
Description
As per #5900 (read that for context), WebKit / Blink already expose it with -webkit-focus-ring-color.
Gecko has these internally as well (-moz-accent-color / -moz-accent-color-foreground).
Depending on how we go about issues like whatwg/html#7993, we might need a standard version of this, but I think that it'd be useful to authors even without it.