RE: Base URIs vs. Document URIs.

There was a similar difficulty in dealing with relative
references inside multipart/related messages that didn't
have a clear "Base URI" either. RFC 2557 made up
a new URI scheme "thismessage" merely for the purpose
of explaining the algorithm for resolution; the 'base'
of a message in transmission was to be considered
"thismessage:/"

You might want to do the same thing for the document URI,
e.g., make up a new URI scheme "thisdocument" whose only
purpose is to treat HREF="#fragment" as resolving to
HREF="thisdocument:/#fragment".