This is a very bulky way of doing this, and doesn't seem sustainable to
implement every shorthand property this way, but it's a place to start.
The "margin" CSS property now expands into its four longhands as far as
my understanding of the specs.
Note that shorthand expansion happens when we *resolve* style, not when
we parse CSS. I'm not sure this is correct anymore, I think other UA's
may actually expand shorthands into the declaration directly at parse
these days. If so, we should do this at parsing as well.
It should be possible for the CSS parser to fail, and we'll know it
failed if it returns nullptr. Returning RefPtr's makes it actually
possible to return nullptr. :^)