Per the current spec, authors can set the apply-author-styles flag to true to allow CSS selectors from the document to match content in the shadow sub tree. However, the spec does not call out how this impacts the application of sibling selectors (and nth-child selectors, etc.) for direct children of the ShadowRoot. 

Specifically, are the children of the ShadowRoot treated as an independent collection from the children of the shadow host for selector matching? This seems to be the implied effect, though alternatives like treating the ShadowRoot children as occurring before or after the children of the shadow host could be considered.

Either way, the spec should probably explicitly define this behavior.