[css-values] note counter-intuitive interpretation of unitless 0 inside calc()

[mgiuffrida]

Section 8.1.2 **calc() Type Checking does not explicitly indicate how a unitless 0 inside a calc() expression is treated. For example:

left: calc(0 - 5px);

It's not clear whether this expression is invalid. Inside calc((), the types of arguments to the - operator must be compatible, and the rhs is of type dimension<length>. But what is the type of the lhs: number or dimension<length>?

Since section 5 says:

for zero lengths the unit identifier is optional (i.e. can be syntactically represented as the <number> '0'

I would argue that interpreting 0 as being of type dimension<length> would be valid, and would be more in line with developer expectations. However, this was discussed in 2010 with @tabatkins agreeing the 0 should cause a parse error. That's fine, but it would be helpful if the fact that 0 is never treated as a dimension inside calc() were explicitly noted in the spec (which was also agreed upon in that discussion).

[tabatkins]

It's invalid. Note the spec text:

<number-token>s are of type <number> or <integer>.

I've gone ahead and added a note about this in 1024dc3.