CSS-Typed-OM has the following hierarchy:
CSSStyleValue
- CSSNumericValue
- CSSUnitValue
- CSSMathValue
- ...various math functions...
Somewhat unintuitively, numbers are also stored using CSSUnitValue with
`unit = "number"`.
There are no distinct classes for LengthStyleValue, etc in the spec, but
they're convenient for us, so they are implemented as subclasses of
CSSUnitValue, at least for now.
(cherry picked from commit 895276426791567f22a612c4a40fa177571cc03e)
We have double precision in the parser, and currently use doubles for
most of layout, so we might as well keep that extra precision inside
NumberStyleValue too.