A variation on the theme would be to descibe it in terms of deltas from
some preset DTD (W3C's ?) as maintained at some repository. Maintaining
the DTDs as diffs from the base version.
Content can be negotiated based on the version control numbers.
Furthermore this data can be distributed, and it is easy to tell when
you need to update you local copy.
Based on the version trees it it seems possible to derive sensible
fallback strategies.
- Jon Tidswell
Disclaimer: I think my thoughts are my own, and I believe my writings are too.