Perhaps a simplified XML is simply well-formed HTML5. Instead of redesigning namespaces, duplicate/use the success of CSS (and you get inheritance) to build the namespaces which can live along different dimensions. Advocate skinning/styling at the CSS level and behavior at the JS layer. Show the benefits of the separation of concerns. Maybe it would be best to teach the basic semantics of HTML5 using the id, class and rel attributes to their fullest, with best practices. When people can understand one language and not be able to do what they need, they will get frustrated and want to expand/change it. Itch/scratch and eventually you will end up with XML (OK, maybe not using URLs for namespace IDs).
(Wouldn't a CSS-like schema language be cool? In other words, for XML to gain adoption, don't always use XML.)
