[css3-values][css-variables] definition of <value>

For the default value of attr() and the arguments of cycle(), the spec
defines <value> as "CSS value that is valid where the expression is
placed", which, based on some discussions on the CSS Variables spec, is
known to be pretty underspecified (the '!important' issue, how to deal
with shorthands, etc.). I assume the idea is that we put in these
details when there's enough implementation experience at the CR stage.

However, I think it would be better to have a common place for common
questions shared by the three (attr, cycle, var). Whether that's a new
section of this module, a subsection of "Data Types Defined Elsewhere"
having <value> loosely defined and extended/modified by CSS Variables, I
don't know.

In any case, here are two issues that should be addressed here because
they only apply to attr() and cycle(), not var():

1. can <value> contain ',' ? If yes, how is syntax ambiguity resolved
within a cycle() ?

2. they both use "valid where the attr()/cycle() expression is placed".
That's fine, but I think the spec should talk about the situation when
there are multiple attr() or cycle() for a property value: all the
possible combinations should be valid. (Or just disallow this at all.)


Cheers,
Kenny