From IRC:

3:45 PM https://html.spec.whatwg.org/multipage/forms.html#labeled-control 'If the attribute is specified, the attribute's value must be the ID of a labelable element in the same Document as the label element.' Is that true regardless whether the label is in the tree or not?
5:27 PM "in the same Document" should probably refer to https://dom.spec.whatwg.org/#in-a-document in some way so that no one has to wonder about that ever again.

/cc @nox