One issue that came up in #3207 (noticed by @heycam while reviewing code from @emilio) is that the spec should define to what extent namespace prefixes are required to be declared. In particular, how do:

@supports (content: attr(n|href)) {
}

CSS.supports("content", "attr(n|href)");

CSS.supports("(content: attr(n|href))");

depend on whether the namespace prefix n has been declared? Do they:

• treat all namespace prefixes as already declared?
• treat all namespace prefixes as undeclared (invalid)?
• depend on a set of namespace declarations (which set, exactly)?