This is a bit under-specced, specifically there's no definition of
CSSMarginDescriptors so I've gone with CSSStyleProperties for now. Gets
us 17 WPT subtests.
I was wrong when I added those notes before about this being impossible,
it's *very* possible, for example with the `@page margin` descriptor.
However, until we have a large number of these shorthands and not just a
single example, we can get away with hard-coding support for it.
Ideally we'd be able to share the code between page selectors and style
ones, but given how simple page selectors are, some code duplication is
the simpler option.
