See https://lists.w3.org/Archives/Public/www-style/2014Nov/0035.html

Similarly, CSSOM defines Element.pseudo(type) while CSS Pseudo-Elements defines Window.getPseudoElements(elem, type) for what seems to be the same purpose. [Also, I notice that Window.getPseudoElements() takes a type, then returns a list that can be filtered by type which seems odd to me.]

I don't know what the pros and cons of each style of interface is, and I couldn't find any discussion about it in the archives, so I think we still need to decide? Also if we're going with the latter, that parenthetical needs to be addressed.