Which has an optmization if both size of the string being passed
through are FlyStrings, which actually ends up being the case
in some places during selector matching comparing attribute names.
Instead of maintaining more overloads of
Infra::is_ascii_case_insensitive_match, switch
everything over to equals_ignoring_ascii_case instead.
I was wrong when I added those notes before about this being impossible,
it's *very* possible, for example with the `@page margin` descriptor.
However, until we have a large number of these shorthands and not just a
single example, we can get away with hard-coding support for it.
This holds the boilerplate that's needed by any CSSStyleDeclaration
subclass that holds Descriptors. CSSFontFaceDescriptors now only has to
worry about initialization and its own exposed properties.
