It's not a question of implementation power. It's about spec requirements. Until now CSS didn't require more than 1 token to know where we are. Basically, parse

It's not a question of implementation power. It's about spec requirements. Until now CSS didn't require more than 1 token to know where we are. Basically, parser doesn't need to backtrack at all. SCSS apporach, for instance, requires look ahead. Consider: color:hover over some random markup & Currently CSS can be sure whether that is a selector or a property depending where it finds it. If it's in a ruleset or at-rule block, it's a selector. If it's in a declaration list, it's a property-value pair. Now, with nesting it becomes confusing. Parsers can not rely on the state to decide what they're looking at. One way to fix this is, like many suggested here, to require look-ahead from parser. Which can be a solution in a well defined environment. Unfortunately, the Web is not one of them. We have many underpowered kiosk-type devices that probably can not spare arbitrarily large look-ahead buffers. Another solution is to somehow let the parser know what it's looking at. That's why leading & is a good indicator it's a selector. & is not used for anything else in CSS. But for selectors where it's not first we need @nest (or anything else that unambiguously marks a selector). Look-ahead requirements are bad for another reason, too. It makes CSS platform-dependent. Currently any version of CSS clearly states what it provides. Implementations can be partial but they can not claim they support CSS 2.1, for example, when they don't implement 100% of it. Wit look-aheads things become murkier. An implementation can support 100% of the spec but depending on the look-ahead required to parse a stylesheet they might not work. Currently, I don't think there are any CSS features that depend on platform limitations, at least not for parsing. Requiring a look-ahead buffer to parse a stylesheet can be very problematic. Imagine, Twitter got hacked and their embeds started stylesheet have a couple selectors that require 1M tokens look-ahead buffer. How that would break every site that embeds tweets?