We provide a mathematical model that supports the
formal description of visual interfaces’ behaviour. The formalism is based on type-inference notation, in which each variable is defined in the domain of the interface basic widgets, and each
transition from a given state of the interface to the following one is represented by the application of an inference rule. When a sequence of actions is made by the user, the behaviour of the corresponding interface is totally defined by the set of inference rules. This formalism allows the designer to formally verify the
properties of the interface
