This paper applies Lehman's theory of software evolution to analyse the characteristics of web-based applications and identifies the essences and incidents that cause difficulties in developing high quality web-based applications. It is argued that they belong to Lehman's E-type systems, hence satisfy Lehman's eight laws of software evolution. The uncertainties underlying the development of web applications are analyzed and their implications are discussed. In order to support sustainable long term evolution of such systems, we proposed a cooperative multi-agent system approach to support both development and maintenance activities. A prototype system with emphasis on testing and quality assurance is reported. The Web has become a distributed and hypermedia software platform. It has stimulated many new applications [1]. However, developing web applications are complex, difficult and expensive. As a consequence, software quality suffers most. In this paper, we discuss wh...