The old versions were renamed to JS_DECLARE_OLD_NATIVE_FUNCTION and
JS_DEFINE_OLD_NATIVE_FUNCTION, and will be eventually removed once all
native functions were converted to the new format.
Websites being able to query whether we support a given CSS feature
should prevent them from loading unnecessary polyfills for things we
already support! Or at least, that's the nice theory. :^)
This is the `CSS` namespace defined in IDL here:
https://www.w3.org/TR/cssom-1/#namespacedef-css , not to be confused
with our `Web::CSS` namespace. Words are hard.
`CSS.escape()` lets you escape identifiers that can then be used to
create a CSS string.
I've also stubbed out the `CSS.supports()` function.
