The World Wide Web is evolving from a platform for information access into a platform for interactive services. Several applications are already used through Internet and Web browsers. User interface of such an application is defined by HTML. However, HTML has its deficiencies when used as a general UI description language. Several parties have addressed this problem by defining specific UI description languages. Thus, for instance, a web browser could be used as a user interface for any application. We have revised the requirements for a UI description language from literature and evaluated two XML-based UI description formats against the requirements through use cases.