The CSS content property's alt text (after `/`) was incorrectly
accepting any content value. Per the CSS Content Module Level 3 spec,
alt text only accepts <string>, <counter>, and <attr()> values. This
change adds type validation in the alt text parsing branch to reject
URLs, quote keywords, images, and other non-alt-text value types.
This fixes 64 subtests in the content-invalid WPT test.
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.