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
`<syntax>` is a limited subset of the "value definition syntax" used in
CSS specs. It's used for `@property`'s `syntax` descriptor, and for the
`type()` function in `attr()`.
