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.