These are created when a style rule has properties listed after another
rule. For example:
```css
.test {
--a: 1;
--b: 1;
--c: 1;
.thing {
/* ... */
}
/* These are after a rule (.thing) so they're wrapped in a
CSSNestedDeclarations: */
--d: 1;
--e: 1;
--f: 1;
}
```
They're treated like a nested style rule with the exact same selectors
as their containing style rule.
