No. In HTML, both the tag semantics and the tag set are fixed. The W3C, in conjunction with browser vendors and the WWW community, is constantly working to extend the definition of HTML to allow new tags to keep pace with changing technology and to bring variations in presentation (stylesheets) to the Web. However, these changes are always rigidly confined by what the browser vendors have implemented and by the fact that backward compatibility is paramount. In addition, for people who want to disseminate information widely, features supported by only the latest releases of Netscape and Internet Explorer are not useful.
XML specifies neither semantics nor a tag set. In fact XML, is really a meta-language for describing markup languages. In other words, XML provides a facility to define tags and the structural relationships between them. Since there's no predefined tag set, there can't be any preconceived semantics. All of the semantics of an XML document will be defined either by the applications that process them or by stylesheets.