Style sheets, which are used to specify the appearance of documents, are rapidly growing in their importance for the World Wide Web. Cascading Style Sheets are now in widespread use and work on a future Web standard, the Extensible Style Language (XSL), is proceeding at a rapid pace. In this paper, we show how a different style sheet language, PSL, represents an attractive midpoint between CSS and XSL in complexity and power. PSL is based on general language design principles that give it simple syntax, easily-described semantics, and considerable expressive power. PSL is supported by Proteus, a portable style sheet system that allows the construction of novel user interfaces for Web browsers and other presentation tools through support for multiple simultaneous presentations of the same document.