>> >> However there are a lot of things that can go wrong when you work with
>> >> things like css.
>> > I don't think it should be the job of CSS mode to highlight ad-hoc
>> > mistake conditions like that.
>> There's cwarn-mode for something similar, so the question really is
>> whether this error is common or specific to a particular user.
>> I have no idea which it is.
> The "!important" is colored as it is now even when I happen to put it
> after the ";". Not syntax coloring that would be enough to easily
> detect the problem.

That would be perfect, yes. Patch welcome,

I think this is enough:

;;("!\\s-*important" . font-lock-builtin-face)

("[^; \t\n]\\s-*\\(!\\s-*important\\)"

(1 'font-lock-builtin-face))