[css3-values] calc() division with units

Some interesting use cases for calc() currently seem impossible due to the
way units work when dividing values.

For example to (roughly) estimate the height of a long line of text which
has wrapped on a narrow screen, you need to divide the total width of the
line of text by the viewport width. Supposing you know the text contains
237 characters:

number of lines ~= 237ex / 100vw


So if your line-height is 1.2em, you might try to estimate the wrapped
height using:

height: calc(237ex / 100vw * 1.2em)


Mathematically, this is fine, as <length>/<length>*<length> == <length>.

But 8.1.2. Type
Checking<http://dev.w3.org/csswg/css-values/#calc-type-checking> for
'/' requires "check that the right side is <number>", and so this
expression is (unnecessarily) deemed invalid.

Can we relax this requirement, and just require the final computed value to
be of an appropriate type?

Cheers,
John