Traverse history by a delta is unclear for deltas larger than 1

[cbrewster]

The steps for Traverse history by a delta https://html.spec.whatwg.org/multipage/#traverse-the-history-by-a-delta seem to assume only a single browsing context will be traversed.

In the case of a delta > 1, multiple browsing contexts may be traversed to a different document. In this case, it is not clear what the specified browsing context should be.

Should each browsing context that is being traversed prompt its active document to unload?
If so, what should happen if one document is allowed to unload, but not the other?

I have created a page to test this behavior across multiple browsers https://servo-experiments.surge.sh/beforeunload-parent.html

cc @asajeffrey @gterzian