The spec allows for these either to be based on the OS, or to be defined
by the browser. Looking at the other browser engines, there's a mix of
the two options. Since we've had issues with using OS colors as
defaults, let's use hard-coded colors for now. Some of these are based
on the definitions in
https://html.spec.whatwg.org/multipage/rendering.html
Reuse the check from IdentifierStyleValue in the CSS Parser, instead of
duplicating it. This might not be the ideal place to put it, but it
works for now.
This will be needed to access the color of a stop from a SVG gradient
<stop> element (which does not participate in layout, so does not have
a layout node).
