Two font properties, font-feature-settings and font-variation-settings,
contain a list of values that are an `<opentype-tag>` followed by a
single value. This class is intended to fill both roles.
CSS Fonts level 4 renames font-stretch to font-width, with font-stretch
being left as a legacy alias. Unfortunately the other specs have not yet
been updated, so both terms are used in different places.
We previously had 4 single-instance StyleValues for these keywords.
CSS-Typed-OM expects them keywords to be exposed as CSSKeywordValue, so
it's simpler to treat them the same. The single-instance behaviour is
kept by having StyleValue::create() use a cached instance for each of
these.
For a long time, we've used two terms, inconsistently:
- "Identifier" is a spec term, but refers to a sequence of alphanumeric
characters, which may or may not be a keyword. (Keywords are a
subset of all identifiers.)
- "ValueID" is entirely non-spec, and is directly called a "keyword" in
the CSS specs.
So to avoid confusion as much as possible, let's align with the spec
terminology. I've attempted to change variable names as well, but
obviously we use Keywords in a lot of places in LibWeb and so I may
have missed some.
One exception is that I've not renamed "valid-identifiers" in
Properties.json... I'd like to combine that and the "valid-types" array
together eventually, so there's no benefit to doing an extra rename
now.
