In bug 1635675 and bug 1639756 I'm changing our UA sheets (one because I'm adding a web-exposed pseudo-element, another because I'm just rejiggering stuff).
Part of what I did was making some selectors a bit less redundant, like going from:
::placeholder, as the engine knows how to only match placeholder rules in some cases.
This however trips some inspector tests, because now stuff starts matching when it didn't use to, and inspector tries to match every pseudo-element against every element.
This is, in fact, very annoying when you have the "Show browser styles" setting applied, as you get pseudo-elements for all sorts of stuff even though you're just looking at, e.g., a
Inspector could do better and only query pseudo-elements that are known to be useful. This may even reduce the work substantially enough that enabling collection of UA rules by default may be feasible, who knows :)