Context:
https://github.com/whatwg/dom/pull/299/files/82df412250bd1321562cbe50574219860b976645#r75397386
#299 (comment)

There was a partial fix in 6ae89ed, but when two Attr nodes attached to elements in the same tree and when you're comparing an Attr node to an element in the same tree it shouldn't do the pointer comparison, but instead behave as if the Attr nodes are children of its element.

@cdumez