Tagname selectors in CSS can have a namespace prefix supplied. This prefix has to be declared earlier in the stylesheet with a @namespace rule, associating it with a namespace url.

Using a namespace prefix that's not declared currently counts as a *syntax error* in the selector, rather than just not matching anything. This combines with the annoying selector behavior that one invalid selector in a list invalidates the entire list. This all means that you have to carry around the namespace map when *parsing* selectors (not just evaluating them), and infects some other APIs involving selectors.

It would be much more convenient if unknown namespace selectors just caused the selector to not match anything. This should only cause a significant behavior change for cases where there is a selector list, and one of the selectors in the list has an unknown namespace prefix, thus throwing away the other selectors in the list.

Could we add a use-counter for that case? I expect this will be absolutely miniscule.

(setting Chris as owner because he suggested making the bug for this; feel free to reassign as needed)