Usability evaluations should start early in the Web development process and occur repeatedly throughout all stages to ensure the quality of the Web application, not just when the product is completed. This paper presents a Web Usability Model, which is aligned with the SQuaRE standard, to evaluate usability at several stages of a Web development process that follows a Model-Driven Development (MDD) approach. The Web Usability Model is generic and must be operationalized into a concrete MDD method by specifying the relationships between the usability attributes of the Usability Model and the modeling primitives of the specific Web development method. To illustrate the feasibility of the approach, we present a case study where the Usability Model has been applied in the evaluation of the models that are produced during the Web application development process.