Abstract : HTML has popularized the use of style sheets, and the advent of XML has stressed the importance of style as a key area complementing document structure and content. A number of tools are now available for producing HTML and XML documents, but very few are addressing style issues. In this paper we analyze the requirements for style manipulation tools, based on the main features of the CSS language. We discuss methods and techniques that meet these requirements and that can be used to efficiently support web authors in style sheet manipulation. The discussion is illustrated by the recent developments made in the Amaya web authoring environment.