::slotted() can only match the top-level assigned nodes of a slot, not their descendants. Given a flattened tree where <p> is slotted and contains an <em>, which rule (written in the shadow CSS) actually matches, and why?
<!-- light DOM --> <x-q><p>hi <em>there</em></p></x-q>
/* shadow CSS candidates */
::slotted(p) {} /* R1 */
::slotted(em) {} /* R2 */
::slotted(p) em {} /* R3 */