[css-values] Problematic missing length units for 'line-height' and 'tab-size'

[tabatkins]

In many properties, %s are resolved against some other type, and are thus treated like that type in calc(); you can do calc(1% + 1px), for example. There are a small number of properties that do the same thing for numbers; I believe so far it's just 'line-height' and 'tab-size'. Theoretically, we should be able to add these, too - there's nothing theoretically wrong with line-height: calc(1 + 2px);.

In practice, there are some problems. In level 3 calc(), if we allowed it, we'd have to treat "number + length" differently than "number * length" - it's a length-like in the first case, but just a raw number in the second case. In level 4 calc() we'd have a more troublesome ambiguity - is calc( 1 * 1px / 1em ) a "length" (number treated as length-like) or a "number" (number treated as raw number)?

Because of these ambiguities, I'm convinced that we must treat numbers as "number" type in calc(), always, and thus not allow adding them in 'line-height' and 'tab-size'. But that then means that we have a hole in those two properties for no good reason, just because their original syntax used a <number> rather than inventing a special-purpose <length> unit. At some point we probably want to plug those holes, and add two new <length> units for those.

(Tho not interchangeable with lengths, another place where we're using a <number> instead of a unitted value is in 'flex-grow' and 'flex-shrink'. These should have been the fr unit, if we'd caught the discrepancy in time.)