In this paper we present a temporal extension of the World Wide Web based on a complete XML/XSL infrastructure to support valid time. The proposed technique enables the explicit definition of temporal information within HTML/XML documents, whose contents can then be selectively accessed according to their valid time. By acting on a navigation validity context, the proposed solution makes it possible to “travel in time” in a given virtual environment with any XML-compliant browser; this allows, for instance, to cut personalized visit routes for a specific epoch in a virtual museum or a digital historical library, to visualize the evolution of an archaeological site through successives ages, to selectively access past issues of magazines, to browse historical time series (e.g. stock quote archives), etc. The proposed Web extensions have been tested on a demo prototype showing, as application example, the functionalities of a temporal Web museum.