<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:pref='http://test.ru'>
<xsl:output method="html" encoding="UTF-8" version="1.0"/>
<xsl:template match="/">

<b><xsl:value-of select="/pref:test/pref:a"/></b>

</xsl:template>
</xsl:stylesheet>