Many current research efforts address the problem of personalizing the Web experience for each user with respect to user's identity and/or context. In this paper we propose a new high-level model for the specification of Web applications that takes into account the manner in which users interact with the application for supplying appropriate contents or gathering profile data. We therefore consider entire behaviors (rather than single properties) as the smallest information units, allowing for automatic restructuring of application components. For this purpose, a high-level Event-Condition-Action (ECA) paradigm is proposed, which enables capturing arbitrary (and timed) clicking behaviors. Also, the architecture and components of a first prototype implementation are discussed. Key words: Adaptive Hypermedia, User Behavior Modeling, WebML, WBM, Web Modeling, Reactivity on the Web