Before we build any piece of software we would, surely, need to be certain that what we intend to build is what is required by the users of that software. In addition, we would want to be sure that the proposed software will behave as expected, do the right thing under all circumstances and also be usable. These functional and usability requirements are often considered separately. The functional requirements may be considered by way of formal specifications or models which are then subjected to verification and validation to ensure that the required properties hold. Similarly, the usability and user requirements may be captured using task analysis methods and checked by design methods such as GOMS, scenario development and usability testing of incremental prototypes. This paper looks at one way of bringing these two processes together by including the graphical user interface (GUI) design process and design concerns in the formal process of specification and refinement. In particular it considers what it means for a prototype of a GUI to be considered as a formal refinement of a specification. In this paper we will show how we can consider the visual display, or presentation, of the GUI as the refinement, rather than the code which produces that presentation, and how this allows us to apply standard and well-known refinement techniques to informal design artefacts.