XML carries well. All a browser needs to view XML is the data itself and the stylesheet controlling its look. If stricter validation is needed, a description listing out its exact meaning can be used with only slightly more overhead. The solution is here. It's designed to use proven methods and require little retooling to make it a vital part of the Web.
Adoption is intended to be as painless as possible.
XML shall support a wide variety of applications. HTML's reach is widespread. It's independent of hardware or software. People with different platforms can access the same HTML using hundreds of different programs. XML must support a similar or greater range of uses and programs to be successful.