[csswg-drafts] [css-values] Parsing "<integer> | <length>" or "<number> | <length>"

upsuper has just created a new issue for 
https://github.com/w3c/csswg-drafts:

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

Please view or discuss this issue at 
https://github.com/w3c/csswg-drafts/issues/489 using your GitHub 
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