In article <e9ydnTfto-8YB_HZRVnysg@g...>,
Gustaf  <gustafl@a...> wrote:
>My question is: is a fragment ID by itself a URI reference? Is the above 
>example valid, or can xlink:href="#myelement" only refer to the same 
>document?

A URI reference of the form #myelement is always a same-document
reference, regardless of the base URI.

Either by RFC 2396, in which it is handled explicitly without
reference to the base URI:

4.2. Same-document References

   A URI reference that does not contain a URI is a reference to the
   current document.  In other words, an empty URI reference within a
   document is interpreted as a reference to the start of that document,
   and a reference containing only a fragment identifier is a reference
   to the identified fragment of that document.

or by RFC 3986, in which it is handled by using the base URI and comparing
it with the (same!) base URI:

4.4.  Same-Document Reference

   When a URI reference refers to a URI that is, aside from its fragment
   component (if any), identical to the base URI (Section 5.1), that
   reference is called a "same-document" reference.  The most frequent
   examples of same-document references are relative references that are
   empty or include only the number sign ("#") separator followed by a
   fragment identifier.

You can't use xml:base to make a bare fragment refer to some other document.

-- Richard