This paper presents XXL, a new interactive development system for building user interfaces which is based on the concept of textual and visual equivalence. XXL includes an interactive builder and a "small" C compatible special-purpose language that is both interpretable and compilable. The visual builder is able to establish the reverse correspondence between the dynamic objects that it manipulates and their textual descriptions in the original source code. Interactive modifications performed by using the builder result in incremental modifications of the original text. Lastly, XXL not only allows users to specify the widget part of the interface but can also be used to manage various behaviors and to create distributed interfaces.