This paper describes an approach for improving automated usability tool support during the development of websites. Existing usability and accessibility validators only analyse the HTML code of a page when they create a report of possible problems. However, when a web engineering method is used to create a website, additional information is available in the form of models which describe the site. An automated validator can use these models to verify usability guidelines (such as "text on the

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... page should be easy to understand by the target audience") with higher accuracy. It can also perform automatic validation in situations where existent tools only output instructions for manual inspection by the developer. The paper systematically analyses existent guidelines and tools, and identifies ways in which the use of a model can improve verification quality. An extension to existing web engineering models is necessary to support automated checkers. It specifies properties of the users, the technical platform and the environment of use. A flexible approach allows the models to be used by validators running inside an integrated development environment, but also at a later time, without access to the development environment. Finally, the prototype of a model-based automatic usability validator is presented. It features verification of a number of guidelines which cannot be automated by existent validation approaches.