https://drafts.csswg.org/selectors-4/#pseudo-element-structure references a ::shadow pseudo-element, defined in https://www.w3.org/TR/css-scoping-1/#selectordef-shadow

But that's an old working draft from 2014. I don't see ::shadow in the editor's draft https://drafts.csswg.org/css-scoping-1/

So this section may need to be changed.