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.
::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.