The css nesting specification was updated recently and & is not meaningful outside of nested contexts.

When used in the selector of a nested style rule, the nesting selector represents the elements matched by the parent rule. When used in any other context, it represents the same elements as :scope in that context (unless otherwise defined).

specification : https://drafts.csswg.org/css-nesting-1/#selectordef-

example :

<style>
& {}
</style>

This change adds a minimal test for & at the root.