The paper presents the architecture, intended use and implementation of the IRIS Design Support Environment (DSE). This environment supports designers to create inclusive Internet content and applications both by incorporating a wide range of work from the fields of accessibility, usability, user modelling, and content design & adaptation, and also by being in itself highly accessible and usable, capable of being used by designers with special needs. The IRIS DSE is Internet-based, builds on open source frameworks and its modular components are integrated with Web services technology. One of the most important contributions of the IRIS DSE to the current state of the art in existing Web development/authoring tools is its identification of a user-centred approach where content adaptation and user profiling is required. These are implemented with upto-date technologies, adhering to relevant specifications for Web accessibility and incorporating them to the process of designing Internet applications for all.