As mentioned in #5744 and #5748, the factors that affect a page's eligibility to be preserved in BFCache differs between browsers, but some are consistent (e.g. Cache-control: no-store).

I think the current spec allows "preserving" Documents implicitly by having Document objects associated with session history entry, which will be used during history traversal.

I wonder if it makes sense to specify that a Document should never be preserved/re-used on history traversal, or if that falls into "implementation details" territory? The spec change should be minimum, and with that we can remove a lot of ambiguity on "what should happen with this if the Document/browsing context associated with it is no longer active?" on some of the things that will make a page ineligible for bfcache (example).