The CSS box-shadow property takes 2-4 properties that are `<length>`s,
those being:
- offset-x
- offset-y
- blur-radius
- spread-radius
Previously these were resolved directly to concrete Lengths at parse
time, but now they will be parsed as LengthStyleValues and/or
CalculatedStyleValues and be stored that way until styles are later
resolved.
Prior to this commit, PropertyOwningCSSStyleDeclaration::serialized()
did not include custom properties, which lead to an incomplete
`cssRule.cssText` result.
This commit makes that class also serialize the custom properties and
place them before the regular properties in the rule text.
I was not aware of this framework back when implementing this back in
bc54560e59. Add in some basic tests for
this now that we are compliant with the specification.
Although we translate e.g `block` to `block flow` for internal use in
the engine, CSS-DISPLAY-3 tells us to use the short form in
serializations for compatibility reasons.
This adds 9 points to our score on https://html5test.com/ :^)
