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 support of MetroWeb. The main results showed that designers using MetroWeb took into account more usability guidelines and made web pages with less usability errors than designers without MetroWeb.