The current diversity of available devices and form factors increases the need for model-based techniques to support adapting applications from one device to another. Most work on user interface modelling is built around declarative markup languages. Markup languages play a relevant role, not only in the modelling of user interfaces, but also in their implementation. However, the languages used by each community (modellers/developers) have, to a great extent evolved separately. This means that the step from concrete model to final interface becomes needlessly complicated, requiring either compilers or interpreters to bridge this gap. In this paper we compare a modelling language (UsiXML) with several markup implementation languages. We analyse if it is feasible to use the implementation languages as modelling languages.