You have to include a DTD (see http://www.stylusstudio.com/xsllist/200405/post70520.html and answer http://www.stylusstudio.com/xsllist/200405/post90520.html where David Carlisle points this out). The following is correct syntax - you will need to find a DTD
<xsl:output method="xml" indent="yes" encoding="UTF-8"doctype-system="http://foo.org/dont.know.the.dtd"doctype-public="-//Netscape Communications//DTD RSS 0.91//EN"/>