The CSS3 Generated and Replaced Content Module hasn’t been updated in 12 years (EDIT: there’s an Editor’s Draft in the works that doesn’t include the stuff I’m about to mention).
There’s still good stuff in here that nobody’s implementing (like ::outside) - is this being revived somewhere else yet? Maybe scooping it into Pseudo-Elements Level 4?
The Editor’s Draft is being worked on right now; it’s still in a state of flux and isn’t ready for review, but it’s moving. However, all those interesting pseudo-elements are being thrown out, as nobody ever wanted to implement them.
That is quite a shame as ::outside seemed to resolve the “divitis” issue to create complex layouts.
I assume the reason implementers were not interested is the complexity of one DOM element?
Pseudo-elements in general are actually quite complicated to maintain in browser engines; they’re elements that sit alongside the real DOM tree, but can be added or removed at a whim. ::before and ::after are nicely self-contained, as they contain nothing but their content value, but ::outside actually rearranges the layout tree significantly.
I strongly agree that ::outside is nice, but given the lack of interest (and the complexity which pretty much ensures that lack of interest will continue), it’s not worth pursuing right now.