Extensible Stylesheet Language. XSL is used for the transformation of XML-based data into HTML or other presentation formats, for display in a web browser. This is a two-part process. First, the structure of the input XML tree must be transformed into a new tree (e.g., HTML), allowing reordering of the elements, addition of text, and calculations—all without modification to the source document. This process is described by XSLT. Second, XSL-FO (XSL Formatting Objects, an XML vocabulary for formatting) is used for formatting the output, defining areas of the display page and their properties. In this way, the source XML document can be maintained from the perspective of “pure content” and can be separated from the presentation. An XML document can be delivered in different formats to different target audiences by simply switching style sheets.