With the growing proliferation of mobile computing devices, the vision of the web anytime, anywhere and on any device is rapidly becoming a reality. Technologies enabling device-independent presentation and new interaction modalities like voice or gesture are moving from research to commercially available products. As a result, developers are faced with the increasing challenge of providing user interfaces that match the capabilities of the different devices available. Within this paper we present our applicationoriented research that has investigated single authoring of multimodal interfaces on mobile devices. After an overview of related work that explains the motivation behind our approach, we present a prototype authoring tool for the development of graphical as well as multimodal web-based user interfaces for multiple devices. We conclude by discussing the relation of our work to established web markup standards and point out noteworthy issues related to their application within our work.