#197 is part of this, but the problem is deeper.

"the top-level browsing context" is used several times in the spec. However that term is not defined in HTML. Instead HTML defines the top-level browsing context of another browsing context. You need to start with a browsing context to get the top-level one!

So statements like

Valid targets for an implicit root observer include any Element in the top-level browsing context, as well as any Element in any nested browsing context which is in the list of the descendant browsing contexts of the top-level browsing context.

don't make any sense. Whose top-level browsing context? Presumably the intent is not any top-level browsing context, i.e. it's not a valid target to observe a completely different tab...