XSL Formatting Objects (XSL-FO) are the second half of the Extensible Stylesheet Language (XSL). XSL-FO is an XML application that describes how pages will look when presented to a reader. A style sheet uses the XSL transformation language to transform an XML document in a semantic vocabulary into a new XML document that uses the XSL-FO presentational vocabulary. While one can hope that Web browsers will one day know how to directly display data marked up with XSL formatting objects, for now an additional step is necessary in which the output document is further transformed into some other format, such as Adobe's PDF.