Zoomable user interfaces are an evolutionary outgrowth of graphical user interfaces. In this paper we propose a zoomable user interface based on Scalable Vector Graphics. Three-level zooming is proposed as a new paradigm to combine different zooming functionalities in a common interface and support zooming within the window manager. This helps to unify zooming techniques of different applications. To meet the demand of efficient and easy navigation on a user interface, several novel interaction techniques are shown that further support the integration of three-level zooming within the underlying presentation system. For mobile small-screen devices, where the benefit of zooming user interfaces is even higher, the proposed system can be operated with simple pen tap or tap and drag operations. We also present a prototypical implementation, which demonstrates how applications based on the SPARK toolkit can transparently benefit form the proposed technology.