A notation is presented for formally describing Web applicationpsilas design of a user-interface. The function of a Web application is given using formal specification language Object-Z and then some generic classes, relationships and system actions are defined to specify user-interface design. In user-interface design, UAN annotations are added to Object-Z notation. The extended Object-Z notation expresses the presentation relationship between the design and the specification. This formal specification and design is useful in documenting a user-interface design and enables the analysis of the usability and safety of a Web application. In particular, it can be used to generate test cases automatically. An example of student information management system is used to illustrate the notation. Task efficiency is analyzed with the example.