In a time where mobile computing devices and embedded systems gain importance, too much time is spent to reinventing user interfaces for each new device. To enhance future extensibility and reusability of systems and their user interfaces we propose a runtime user interface description language, which can cope with constraints found in embedded systems and mobile computing devices. XML seems to be a suitable tool to do this, when combined with Java. Following the evolution of Java towards XML, it is logical to introduce the concept applied to mobile computing devices and embedded systems.