Introduction: Website accessibility is a problem that affects millions of people with disabilities. While most current accessibility initiatives target government or commercial sites, a growing segment of online content is being created by non-professionals. This content is often inaccessible to users with disabilities. Everyday inclusive Web design is a design perspective that promotes the creation of accessible content by everyday end-users. Method: The design activities of end-users and system designers are modeled using activity theory. Activity models and scenarios are used to describe and analyze design activity. Analysis: Professional system designers and end-users each engage in design activities within social software systems. These activities are not independent, but rather interact with one another to produce the final site content. Results: The activities of both system designers and end-users are mediated by historical practices and by the affordances of the available tools. Accessibility of user-created content may be limited by the ambiguous role of end-user designers, by the invisible impact of inaccessible designs and by the lack of appropriate tools to support end-user accessible design practices. Conclusions: Accessibility of social software systems depends on the cooperative work of system designers and end-users. The underlying structure of social software systems may be altered to increase users' awareness of accessibility issues and to encourage accessible design practices.