(Apologies if this is a little awkwardly stated, it's almost 3 AM and I'm on vacation so I'm trying to keep it brief.)
Some CSS features like ::selection have a whitelist of supported rules to keep them performant and/or secure. (I believe ::selection doesn't support background images because it is considered a hole that would allow easy spying on user activity, and doesn't support many other rules because they would trigger a reflow.)
I think it'd be better if all of these features pulled from common pools of restrictions (ie. all rules that don't support loading assets from a URL have the same fallback behavior).
This would also help manage some of the overly-broad spec amputations made in the name of perf / privacy (eg. disallowing all images when it's really network requests that are the problem).
(This was originally named CSS Privacy & Performance Restriction Modules and filed under CSS, but now that I think about it, these kind of restriction concerns are present in more standards than just CSS.)