Previously we parsed it as `<custom-ident>` in `<counter>` and as a
keyword in `list-style-type`.
The practical effect of this is:
- Spec defined counter style names in `<counter>` are ASCII lowercased
on parse.
- Non spec defined counter style names are allowed in `list-style-type.
We are still to parse the `symbols()` function but this gives us a
better base for that.
