If I am not mistaken, :has(), :nth-*-child(), :current(), host-*(), are the only functional pseudo-classes taking a selector as their arguments, that are not logical pseudo-classes.

I suspect they should pass any restrictions on validity of selectors at their position to their arguments, like logical pseudo-classes. Otherwise, these cases would be valid:

• ::slotted(:has(type)) (valid in FF)
• ::slotted(:nth-child(1 of :hover > :hover)) (valid in Chrome)
• :host(:has(type)) (valid in FF)
• :host(:nth-child(1 of :hover > :hover)) (valid in Chrome)

This is surprising because the following cases are invalid per spec and in Chrome/FF:

• ::slotted(* > type)
• ::slotted(:hover > :hover:nth-child(1))
• :host(type > type)
• :host(:hover > :hover:nth-child(1))

::slotted() and :host-*() take compound selectors, but has() takes relative selectors and :nth-*-child() takes complex selectors.