This work addresses the question of supporting web designers in considering usability in their work in order to foster user-centered design of web sites. With the MetroWeb tool that is described in this paper, designers can access usability guidelines contained in usability knowledge bases, and use them to design a particular web site based on these usability guidelines. MetroWeb consists of a Java-based application helping usability experts to gather usability guidelines coming from different sources and to organize them in a structured way. It then provides designers with guidance in using these guidelines according to a semantic network of concepts structured around the notion of guideline, such as ergonomic criteria, development phase, bibliographic reference, type of web site, type of web page, etc. A first experiment was conducted with professional web designers in order to evaluate their appreciation of the MetroWeb tool. They had to create two web pages with or without the sup...