Typically there is some resource that corresponds to a URI, that is dereferenced and used. XML is a simplification and extension of SGML that has an initial low entry cost, and more complex, but cool, functionality. HTTP sends the whole resource even if the user only wants to look at the fragment. The browser, knowing it is an HTML file, does the appropriate thing, in this case pages to the correct portion of the resource.
Interests try to bias the architecture to their own interest.
AvantoGo uses a conduit to make PDA accessible content unavailable on the Web. You probably can't find it.