Custom styles are specified in the XSL Formatting Objects standard. The purpose of XSL-FO is to provide a mechanism for formatting XML data for print, screen, and other output media. XSL-FO, also known simply as XSL, is a specification of the World Wide Web Consortium and is closely related to XSLT. However, whereas XSLT is most often used for transforming XML into HTML or other XML structures, XSL-FO is most often used for formatting XML for print.