Ampersand syntax looks nice, but how will it handle nested @rules? Or are nested @rules—like @media—not supported?
Nothing special needs to be done for nested at-rules. As I said before, the sole grammatical ambiguity is that a selector starting with a tagname followed by a pseudo-class looks like a declaration. Anything else is fine, including at-rules, which start with an at-keyword token. (To handle nested at-rules just needs some additions to the CSSOM, to add a
.childRules property to the CSSStyleRule interface.)
I do however feel that the ampersand syntax creates inconsistencies. Depending on whether you want to pre- or suffix your selector, you'll have to use the @nest rule. A nesting block prevents this, be it curlies or another character.
@nest rule will allow you to use any nested rule, whether the
& appears at the start or elsewhere, so if you want a simpler model, feel free to use it all the time. (It's identical to the Nesting Block, after all, just with the five extra characters of "@nest".) The idea is just that, in the common case where you start with
&, you don't need to pay the extra cost of the wrapping block.