:nth-child and related selectors should work on the root element. They do in browsers, and according to CSS selector spec, "It is not required to have a parent."[1].
That makes sense to me. But we should check with @ryancox since he's the one that put in those checks that you're taking out, and see what he has to say.
:nth-child
and related selectors should work on the root element. They do in browsers, and according to CSS selector spec, "It is not required to have a parent."[1].[1] https://drafts.csswg.org/selectors-3/#nth-child-pseudo