There is an increasing demand in making web user interfaces richer and more interactive. Currently, there are two approaches aiming at improving web user interfaces. First, downloaded code in form of Java Applet or ActiveX can be executed in browsers. And more recently, AJAX (Asynchronous JavaScript + XML) leverages browsers' JavaScript engine to render user interfaces without reloading pages. Both approaches have some weaknesses.In this paper, we present an alternative approach to creating highly interactive web user interfaces. Our approach is based on the Extensible User Interface Protocol (XUP), a SOAP-based protocol for communicating events and incremental user interface updates on the web. On top of XUP, we have built a web user interface development framework, OpenXUP, consisting of a thin client and a server toolkit which offers a set of event-driven APIs. The framework allows for the rapid development of highly interactive web applications and services.