We were rejecting perfectly valid z-index values like '1000' since we
were passing all CSS values through the length parser and unit-less
lengths are not valid in this context.
It's yet another hack for the ad-hoc CSS parser (its days are numbered)
but this makes the top header links on google.com actually work. :^)
...{All} to ParentNode. Exposes createDocumentFragment and
createComment on Document. Stubs out the document.body setter.
Also adds ParentNode back :^).
