When serializing CSS declarations we now support combining multiple
properties into a single shorthand property in some cases.
This comes with a healthy dose of FIXMEs, including work to be done
around supporting:
- Nested shorthands (e.g. background, border, etc)
- Shorthands which aren't represented by the ShorthandStyleValue type
- Subproperties pending substitution
This gains us a bunch of new test passes, both for WPT and in-tree
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