There really isn't an official XML Schema for XHTML yet.

So you need to use a DTD.

If you want XHTML, put this in your stylesheet:

<xsl:output 
   method="xml" 
   doctype-public="-//W3C//DTD XHTML 1.1//EN"
   doctype-system="http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"/>

If you want HTML, put this in your stylesheet:

<xsl:output 
   method="html" 
   doctype-public="-//W3C//DTD XHTML 1.1//EN"
   doctype-system="http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"/>