1. A method for implementing a Web application comprising: establishing a session for a Web application between a Web server of a Web domain and a browser of a client of a local domain, wherein the Web application is constructed so that view and controller functionality are provided by components executing in the Web domain and where model functionality is provided by components executing in the local domain; during the session, serving an expeditor runtime component from the Web domain to the local domain for the Web application session, wherein a running instance of the expeditor runtime component is referred to as the expeditor runtime, which provides the model functions of the Web application; during the session, serving at least one widget from the Web domain to the local domain, wherein the widget executes within an instance of the expeditor runtime; and conducting the session, wherein end-user data utilized by the Web application is isolated within the local domain and is not exposed to the Web domain, wherein model functions of the expeditor runtime running in the local domain handle processing of the end-user data.