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For example, some things only accept positive numbers. If you have "attr(foo number)", you don't know at parse time whether the number will end up being correct.
You don't know at parse time whether it's even a number, nevermind a positive number. This is already handled; that's why attr() takes a fallback. # The optional <fallback> argument represents a fallback value, which is used # if the named attribute is missing, or its value cannot be parsed into the # given type or is invalid/out-of-range for the property. If it's absent, the # default value for the given <type-or-unit> (from the list below) is implied.
Oh, duh. Don't know why I forgot about that.