The lack of adequate Accessibility of Web sites remains a problem for blind users and other people with disabilities. The process of engineering accessible Web applications involves several issues, which include content, presentation and pragmatic issues that, fortunately, are currently addressed by some standard models such as the W3C's Accessibility guidelines. In this context, our work proposes an approach to better model Accessibility as a concern specified at different abstraction levels. This paper introduces the approach and illustrates its use through a case study, which includes specific guidelines and techniques.