Consumer-centered software applications nowadays are required to be available both as mobile and desktop versions. However, the app design is frequently made only for one of the two (i.e., mobile first or web first) while missing an appropriate design for the other (which, in turn, simply mimics the interaction of the first one). This results into poor quality of the interaction on one or the other platform. Current solutions would require different designs, to be realized through different design methods and tools, and that may require to double development and maintenance costs. In order to mitigate such an issue, this paper proposes a novel approach that supports the design of both web and mobile applications at once. Starting from a unique requirement and business specification, where web– and mobile–specific aspects are captured through tagging, we derive a platform-independent design of the system specified in IFML. This model is subsequently refined and detailed for the two platforms, and used to automatically generate both the web and mobile versions. If more precise interactions are needed for the mobile part, a blending with MobML, a mobile-specific modeling language, is devised. Full traceability of the relations between artifacts is granted.