I get your point. However it relies on an example that actually requires an unbounded lookeahead. This is where I'm confused. Sure, these ambiguous blocks (I'm not sure how many examples there are other than color:hover) aren't prefixed, but they are conclusively postfixed. So any lookahead required should be easily bounded. Where does the unbounded aspect come into it?

Reposting my earlier painfully phone-typed regex to illustrate the point: https://regex101.com/r/aOc2Pz/1

Edit: I should add that I don't doubt that those involved know what they're talking about. I know nothing of briwser-level CSS parsing. I am certain it is a nightmare. I would really just like a better example to explain the unbounded aspect. Especially when it is being given as the primary excuse to cripple / greatly obtusify a syntax I'm being asked for an opinion on. Maybe it's just me but I would expect that an evolution of this magnitude would afford a bit more compromise elsewhere.