[csswg-drafts] [selectors-4] are at-rules allowed to nest inside of style rules

jonathantneal has just created a new issue for https://github.com/w3c/csswg-drafts:

== [selectors-4] are at-rules allowed to nest inside of style rules ==
Are at-rules allowed to nest inside of style rules, per spec? If so, where is this defined?

Example usage:

```pcss
body {
  margin: 20px;

    @media (min-width: 30em) {
        margin: 0;
    }
}
```

I’m asking because folks confuse this with being part of the nesting proposal. And, for my own part, I didn’t help disambiguate this (jonathantneal/postcss-nesting#24). 

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/2752 using your GitHub account