> How do I include an XHTML DTD in my output?

XSLT supports this with doctype-public and doctype-system attributes on
xsl:output. In your case: 

<xsl:output method="xml" doctype-public="-//W3C//DTD XHTML 1.0
Transitional//EN" doctype-system="DTD/xhtml1-transitional.dtd"/>

Linda

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list