XSL (eXtensible Stylesheet Language) is a general term that can encompass XSLT (eXtensible Stylesheet Language Transformations), XPATH (a language for navigating XML data), and XSL-FO.  When working with .xsl documents, the text inside is written in the XSLT language.  In XF Designer 2011, given an XML database, you can "Assign" an XSLT Stylesheet and then "Run XSLT". (Similarly, you can "Assign XML" to an opened .xsl file.)

 

The XSLT file is a language that transforms XML into XSL-FO.  The XSL-FO can then be displayed in a What-You-See-Is-What-You-Get (WYSIWIG) environment by using the Design View of XF Designer 2011.  Because XSLT is a language for transforming XML, it does not lend itself to a WYSIWYG display.  Also note, XSLT for producing HTML will be different than XSLT for creating other documents such as PDF for many reasons, including the fact that HTML does not have a concept of page breaks.  Ecrion can help you converting pre-existing XSLT for use with new document type output formats if this is necessary.  Contact support@ecrion.com for assistance.