bug#25525: 25.1.90; add color highlighting to css mode

[Tom Tromey]

>>>>> "Eli" == Eli Zaretskii <eliz@gnu.org> writes:

Eli> For example, tty-colors.el already has a (longer) list of colors with
Eli> suitable RGB values, so perhaps we should simply add the few missing
Eli> ones to that list, and then use that for CSS?

CSS specifies the named colors that are available -- so the list in
tty-colors.el would not be correct.

Eli> As another example, tty-colors.el also includes code for parsing and
Eli> converting color values (although perhaps not all of the formats you
Eli> support in your code).

Here too the new code supports what CSS specifies.  It's important to
the CSS developer that the Emacs mode follow CSS as precisely as
possible; using other formats supplied by tty-colors.el would result in
code that is invalid CSS.

I think the only area of overlap is the #RRGGBB hex syntax, but that is
so trivial as not to need any sharing.

Eli> As yet another example of existing functionality that you could
Eli> perhaps reuse, there's the :distant-foreground attribute of a face
Eli> that might help you with the issue of color contrast.

It seemed simpler to always set the :foreground, but I can experiment
with :distant-foreground instead.

Tom