[css3-values] Overly limited attr(foo color) syntax?

The parsing of the "color" type in attr() restricts it to being a HASH
or IDENT token - in other words, a hash color or color keyword.  This
disallows the various color functions.  Is this intentional?  If so,
is it desirable?  I'm assuming the answers to both of these questions
are "no".

If I'm right, then we should change the "color" type to also accept a
FUNCTION token, followed by anything, ending with a ) token, and still
require it to parse into a <color> type.

~TJ