[css-values] Parsing "<integer> | <length>" or "<number> | <length>" #489

From #460 I noticed that tab-size's syntax is <integer> | <length>, and I was confused on how 0 should be handled in this case, since 0 is valid as both <integer> and <length>. It seems the same issue applies to <number>, which line-height uses.

I do not find the answer whether for whether 0 should be parsed as <integer> or <length> in this case from css-values spec. Am I missing something, or probably the spec should say something about this case?

cc @fantasai @tabatkins