Some disabled tests:
- We don't implement <transform-list> serialization.
- We don't serialize colors according to spec & as Gecko does.
- We don't round floats correctly (see rust-cssparser/#173).
- We don't implement <resolution>.

It's not clear whether or not we actually need initialValue (see css-houdini-drafts/#286).

Part 13 of a series of patches to implement the CSS Properties & Values API.