Fwiw, while I deeply appreciate WebKit supporting the proposal as well, the editors of the Nesting spec are Googlers and the Chrome impl happened at the same time.
Depending on the exact parsing strategy it might be genuinely required (such as if the parser switches from "property mode" to "rule mode" at some point), but in this case it was just because it's kinda confusing to do so.
We removed the restriction, but still warn against it due to readability issues.