Add an abstract image style value for image-set() and parse both the
standard and -webkit-prefixed spellings through the existing <image>
value path. The parser accepts URL and string image candidates,
optional resolution descriptors, and type() filters.
Track attr-taint through substituted component values so image-set()
candidates using attr()-derived URL-producing tokens are rejected when
resolved for URL-using properties.
Update the relevant WPT baselines now that image-set() parsing is
supported in additional value contexts.
Compare `Vector<Parser::ComponentValue>` directly instead of
serializing them into strings first.
This is required for the upcoming changes where we would compare
previous and new sets of custom properties to figure out whether we need
to invalidate descendant elements. Without this change `equals()` would
show up being hot in profiles.
Treating these like any other ComponentValue means not having to convert
between different types of Vector, and that we will be able to use
TokenStream to parse the "argument grammars" of arbitrary substitution
functions.
