[css3-values] value syntax

I think we should copy over the definition of our syntax notation in CSS2.1
section 1.4.2.1 <http://www.w3.org/TR/CSS21/about.html#property-defs> into
the CSS3 Values and Units spec... and add a definition for a '&&' combinator
which is equivalent to '||' except both sides are required. This will make
it easier to describe various compound value syntaxes.

~fantasai