[variables] invalid variables

Hi all,

Reading though the new variables spec (
http://dev.w3.org/csswg/css-variables/), section 3.1, and examples 7 and 8
specify that a property that is invalid at computed value time must
evaluate to the property's initial value. IMHO this should rather be
specified that the property is ignored (in line with invalid values in
http://www.w3.org/TR/css3-values/) or perhaps evaluate to its default
value, instead. (Inasmuch I think that using 'background-color' in those
example is a bit unfortunate, as it'd evaluate to 'transparent'.)

Apart from giving surprising results (IMHO), evaluating to the initial
value would also prevent use cases like the following:

.foo {
  color: red; // default value
  color: data(override-color); // override only if declared
}

.bar {
  background-color: black;
  background-color: data(bg-color);
  background-color: linear-gradient(from top left to bottom right,
data(from-color) 0%, data(to-color) 100%);
}


Cheers,

- Roland