I'm trying to convert one XML document into another wsing XSLT

XSLT:

<?xml version="1.0" encoding="UTF-8" ?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml"/>
<xsl:template match="/">
<base>
<opis> Typek </opis>
<aa>
<xsl:value-of select="a1/first" />
</aa>
<bb>
<xsl:value-of select="a1/last" />
</bb>
</base>
</xsl:template>
</xsl:stylesheet>

XML doc

<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="a12b.xslt"?>

<a1>
<first>Gerek</first>
<last>Moszunsi</last>
</a1>

Alas IE6 is displaying HTML output -- not XML

Please help me! What should I do to get XML document as a result of transformation