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.
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