To facilitate wide adoption of wireless devices for Internet access, it is important to enable the "develop once and use everywhere" trademark. This means that a Web page can be developed in HTML for presentation on desktop machines, and then be automatically adapted and displayed on any device including handheld devices. In this paper, we propose a mediator Web service that automatically adapts Web content into suitable formats based on receiving contexts. Our underpinning is a hierarchical semantic meta model and a semantic ontology that guide in partitioning a Web page into semantic segments, which refer to atomic units encapsulating one or more of the four types of semantically coherent elements in a content page: function (functionality related), typesetting (readability related), presentation relationship (space and time related), and literary context (semantics related). We also report our experiences in designing and implementing the mediator service. Our experimental results demonstrate that our service is effective and efficient in automatically transforming Web content into semantic-retained formats suitable to be displayed on handheld devices.