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https://drafts.csswg.org/cssom/#extensions-to-the-element-interface [[ partial interface Element { PseudoElement? pseudo(DOMString pseudoElt); }; ]] It seems this API lacks implementor interest. Remove.
https://github.com/w3c/csswg-drafts/commit/53fe1426eb63101b087286c65b10a26d9073a943
For what it's worth, we implement CSSPseudoElement in Gecko but only so we can identify animations whose target is a pseudo element. https://dxr.mozilla.org/mozilla-central/source/dom/webidl/CSSPseudoElement.webidl
OK, that interface is defined at https://drafts.csswg.org/css-pseudo-4/#csspseudoelement