The presentation of the content of an XML file (or XML files) is specified by one or more XSL (eXtensible Style Sheet Language) files. Both files are processed by an XSLT (eXtensible Style Sheet Language Transformation) processor. The output of the XSLT processor may include HTML documents or other kinds of presentation media.
This means that from the same XML file describing the content, two (or more) XSL files can describe how to generate two different HTML documents from the same source content document. One XSL specifies a visual or media-rich fancy document, while another specifies an ADA compliant document.
It is out of the scope of this module to discuss XML, XSL and XSLT. These topics require a whole course. In the long run, however, it may save a web author time and money. Also, note that an XML file does not need to be static. It can be dynamically generated by a server-side script.