More efficient how much?
It's really less about benchmarking and more about managing risk: large unbounded lookahead in scss might crash a single concurrent pod in your CICD - isolated, detectable and really easily remedied. The same here could crash many users' browsers - not so easily fixed.
I feel like we're talking serious edge cases here.
I'm not sure I can think of a better explanation/examples than the gp. It essentially comes down to ambiguity between single element selector names (which are unprefixed in CSS - no dot or hash) and property keys (also plain identifiers).
The only similar nested braces form we have in current CSS is always @-prefixed. There's no unprefixed equivalent for parent blocks in current syntax (plus those blocks are currently limited to a single level which I guess mightt simplify things further given the prefixed selector is always at root level)
> I also don't see why a dramatic update like this couldn't also introduce / require a 'strict' style to be considered valid.
I think it could and I think the call for input is open to that as a possibility. The above comment is just referring to a fully compatible direct scss implementation of nesting.
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.
The advantage of only looking ahead by a constant is that parsing can be done in linear time. For a backtracking parser, you have to look ahead by up to n characters each time, leading to a quadratic runtime in the worst case.
Seriously tho it seems like a pretty major feature to be introducing. Why not do it right? Surely the complexity should be concealed rather than out front being dealt with in user space.