XML has been proposed in order to bring to the web a markup language free of the shortcomings of HTML, in particular the inextensibility of the set of valid elements (tags). Stylesheet languages have been proposed for XML, in order to provide precise and sophisticated typographical control over the appearance of text-based data. We have developed a rendering engine for HTML and XML documents, providing rudimentary support for typography, but allowing easy extensions (displets) for any kind of data, including non-textual ones, such as math, charts, graphs, etc. Some extensions have already been developed: here we present the one for supporting Z, a notation for formal specifications of software systems. Keywords XML, Markup languages, XSL, Stylesheet Languages, Displets