[css-conditional-?][css-values-5] @supports should not use argument grammar

[andruud]

(Originally reported by @bramus elsewhere.)

The argument grammar of a function is its "basic shape" that determines whether or not it's considered valid during parsing. For example, color: if(potato(yes):green) is valid parse time, even when potato() is not a supported if-test.

Currently, @supports (color: if(potato(yes):green)) {} just checks whether or not the declaration would parse as normal, incorrectly suggesting that potato() is a supported if-test.

We should probably say that @supports checks the standard grammar instead, though we need to ensure that "var()-in-var()" cases are still reported as true, e.g.: @supports (color: if(var(--my-test):green)) {}.

See also #7280, where we decided that selectors should not be forgiving for @supports.

[cdoublev]

So @supports (color: var(var(--name))) {} should evaluate to false?

(edit) you covered this with "we need to ensure that "var()-in-var()" cases are still reported as true", sorry for this hasty comment.

[tabatkins]

Is the idea that if there are no substitution functions used in the test, we check the full grammar; otherwise we check the argument grammar (since it's all we can do)?

[andruud]

Yes, I think that behavior sounds sensible? (Can we do better?)