I think this is a good idea, but your language seems extremely similar to XML - why not just write an XML schema? Seems like you can leverage a lot of existing tools and abstractions that way.
SrcML is a piece of prior art that we studied that does use XML and I think it limits them. For example because literal content isn't quoted in XML all content inside tags is inner text. So they can't pretty print their documents, because indentation added to the embedding document would become indentation in the embedded document instead. Oops! We also support named references and named namespaces, which XML does not.
