I'm not a real doofus, but I play one at a national laboratory.
Style sheets allow information about presentation to be kept separate from the content.
This allows more flexible and efficient Website maintenance, makes Web pages more accessible, and makes the Web faster. W3C will be developing both the XSL and CSS style sheet languages in parallel, as they are both useful for Web sites and they give Web designers an expanded set of tools to do their work. CSS is used to style HTML and XML documents on the Web. In addition to styling XML documents, XSL is also able to generate new XML documents from XML data. XSL and CSS will share the same underlying concepts and will use the same terminology as much as possible.