The definition of syntax in the "css-properties-values-api" spec (which
is used for the `@property/syntax` descriptor) is slightly different
from the definition of `<syntax>` in the "css-values" spec (which we
implement) in that it limits literal idents to exclusively
`<custom-ident>`s (i.e. not CSS-wide keywords or "default").
`<custom-ident>`s are also case-sensitive so that behavior is
implemented for syntax matching here as well
If a custom property is registered, then it may have an initial value
which we should fall back to here.
This is a temporary hack until we cascade our custom properties (see
https://github.com/LadybirdBrowser/ladybird/pull/6608).
