[css-values-4] Allow combining number and percentage

[cdoublev]

In CSS Values 4, 10.9. Type Checking:

Note: In previous versions of this specification, multiplication and division were limited in what arguments they could take, to avoid producing more complex intermediate results (such as 1px * 1em, which is <length>²) and to make division-by-zero detectable at parse time. This version now relaxes those restrictions.

[...]

Note that <percentage>s relative to <number>s, such as in opacity, are not combinable with those numbers — opacity: calc(.25 + 25%) is invalid. Allowing this causes significant problems with "unit algebra" (allowing multiplication/division of <dimension>s), and in every case so far, doesn’t provide any new functionality. (For example, opacity: 25% is identical to opacity: .25; it’s just a trivial syntax transform.)

Is the second note obsolete or is there a reason not to allow combining <percentage> and <number> when <percentage> is resolved relative to <number>?

[tabatkins]

The note is still valid, and in fact was written after adapting calc() to handle unit algebra. Allowing %s to resolve to an empty type (like numbers) would make a lot of definitely-invalid calculations appear possibly-valid, and you wouldn't know they aren't until you actually resolved the percentage's type.