The XSL output method determines the default MIME type, and also some aspects of syntax in serialized tags:
An explicit
<xsl:output method="..."/>in the stylesheetOtherwise, the default is HTML
It is not the usual XSL default of XML. If you want XML then make it explicit in the stylesheet
The XML declaration is written by default, for output method XML. This is appropriate for a complete XML document written from a single transform. If the XML output is written as a series of nested fragments, then the XML declaration must be disabled for the embedded output using an extra flag on the <xsl:output> element. For example:
<xsl:output method="xml" omitxmldeclaration="yes"/>
