This document discusses the Quill tool, which aims to simplify the development of cross-platform web user interfaces through collaborative model-based design. It presents the motivations for such an approach due to the fragmented device market and heterogeneous environments. A survey of IT practitioners found that while they recognize the benefits of models, context-awareness and adaptation, they do not fully incorporate these into their work. Quill is proposed as a web-based, open source design assistant that facilitates flexible, cross-platform design through models hosted in the cloud and collaborative editing. A case study demonstrates how Quill can be used to design a task model, abstract and concrete UIs, and the final UI design.