HTML defines :defined: https://html.spec.whatwg.org/multipage/semantics-other.html#selector-defined.

1. Is that enough for a definition of a new pseudo-class?
2. Is it fine for HTML to define this?
3. Should Selectors contain a registry of externally defined selectors?

(I'm asking this in part since we're considering a :heading(level) pseudo-class.)

See also: WICG/webcomponents#665.

cc @domenic @snuggs