I believe this goes against the official specification of XML
And while that sounds like just a funny gimmick, it has real practical applications:
If you've got a CMS that generates HTML from XML documents, you can write the XSLT for that by hand of course. But if there are common patterns that most sites use (menus, for example), while different customers use their own custom document format, it would be really nice if you could generate that XSLT from the data model definition. Long ago I've worked on a CMS that did exactly that.