For a while, WebKit-derived browsers have had a quirk where all ::-webkit-* pseudo-elements are considered valid at parse time (and thus won't invalidate a selector list if they appear in only one selector). This has apparently begun to create compat problems for Firefox (see whatwg/compat#103), so they're adding that behavior themselves. This behavior is being tracked in WPT tests in web-platform-tests/wpt#12673.

We should go ahead and inline that into the Selectors spec, probably in a compatibility appendix.