Background is the discussion in #4437.

As of that change, KeyframeEffect now has a pseudoElement member whose syntax is defined by Selectors Level 4.

That definition requires UAs to accept the old single colon syntax (:before) as well as the double colon syntax (::before).

I was under the impression we wanted to phase that historical artifact out of the web platform so that new APIs like this should reject :before and only accept ::before.