Mashups propose a lightweight integration of applications by combining distributed web resources. Their development is aided by a number of visual tools and platforms. However, they all imply a server-side mashup execution. Since data, application logic, and UI can all be provided as a service, we argue that the client can be the ideal integration platform for such applications. In this paper we highlight challenges and requirements of such a thin-server composition and execution system and propose a corresponding architectural concept to integrate back-end as well as front-end services of a composite application within the browser. We further present an implementation of our concept and validate its practicability by means of a sample application.