"Functional" as in "it's a function token" and not "it works", because
the behaviour for these is unimplemented. :^)
This is modeled after the pseudo-class parsing, but with some changes
based on things I don't like about that implementation. I've
implemented the `<pt-name-selector>` parameter used by view-transitions
for now, but nothing else.
Pseudo-elements have specific rules about which CSS properties can be
applied to them. This is a first step to supporting that.
- If a property whitelist isn't present, all properties are allowed.
- Properties are named as in CSS.
- Names of property groups are prefixed with `#`, which makes this match
the spec more clearly. These groups are implemented directly in the
code generator for now.
- Any property name beginning with "FIXME:" is ignored, so we can mark
properties we don't implement yet.
We previously supported a few -webkit vendor-prefixed pseudo-elements.
This patch adds those back, along with -moz equivalents, by aliasing
them to standard ones. They behave identically, except for serializing
with their original name, just like for unrecognized -webkit
pseudo-elements.
It's likely to be a while before the forms spec settles and authors
start using the new pseudo-elements, so until then, we can still make
use of styles they've written for the non-standard ones.
