This is the final CSSStyleValue class used by the per-property test
harness, so those now actually run instead of throwing an exception on
load. 🎉
+39 WPT subtests. (Plus however many from the per-property tests finally
running.)
The two failing serialization tests are also failed by Safari in exactly
the same way, so that seems more like a spec issue. (The spec is
incomplete in quite a few places.) The failing subtest for toMatrix() is
also a spec issue: is2D is handled oddly by CSSMatrixComponent and this
subtest fails because of the `matrix` getter, which is unspecified. See
https://github.com/w3c/css-houdini-drafts/issues/1155 for details.
