Allow CSS Custom Property in mask-icon's color attribute

[jfbrennan]

What problem are you trying to solve?

Avoid hardcoding a color value in HTML and having to maintain that value alongside its source of truth in CSS.

What solutions exist today?

None. You have to hardcode the color value.

How would you solve it?

I would support CSS Custom Property as a valid value for <link rel="mask-icon" color="--my-color" ...>. Today the color attribute supports hex, rgb, or the built-in color keywords.

Anything else?

WebKit said come here (https://bugs.webkit.org/show_bug.cgi?id=266440), so here it is.

cc @annevk

[zcorpan]

cc @whatwg/css