Acceptance tests are usually created by a client after a part of a system is implemented. However, some methodologies propose elaborating test cases before the implementation of a system. This approach increases the probability of implementing a system that fulfills requirements, but may be problematic for customers and testers. To allow acceptance testing in such conditions we propose to define test cases by recording them on an interactive mockup (a low detailed user-interface prototype). The paper focuses on Test Description Language, a notation used to store test cases.