The current grammar for CSS @namespace rules is:

@namespace <namespace-prefix>? [ <string> | <url> ] ;

Since it uses the <url> production, that means any URL modifiers defined, such as the request modifiers in CSS Values 5 should also parse.

I don't think it's meaningful for @namespace to have request modifiers (though I may be mistaken). I propose adding some way for specifications like @namespace to parse only the URL itself.

(cc @noamr, @annevk, @tabatkins, @fantasai)