Implementation and spec finalisation are fairly different things these days—in fact, in at least some parts of the web platform (JS), the spec is not permitted

Implementation and spec finalisation are fairly different things these days—in fact, in at least some parts of the web platform (JS), the spec is not permitted to be finalised until two implementations are shipping. Firefox hasn’t shipped :has() for technical reasons, not because :has() isn’t ready. (Notwithstanding all this, it is bothersome when incomplete implementations are shipped, which is happening fairly regularly in at least small ways and has already happened with :has() with Safari having to fix important bugs and spec changes after their initial release of it. This kind of stuff often makes feature detection much harder and sometimes impossible.)