You can use an attribute like this:
<a href="http://www.esri.com" target="_blank">http://www.esri.com</a>
Attached is a layer package that shows the various attribute and how they behave.
The XSL Template was modified in the
<xsl:template match="Field"> Section.
if you look at line 1 below it was modified to show attributes that start with HTML tags to display as HTML.
Defatault XSL template.
<xsl:when test="FieldValue[starts-with(., '<img ')]">
<xsl:value-of select="FieldValue" disable-output-escaping="yes" />
</xsl:when>
Modification to template.
<xsl:when test="FieldValue[starts-with(., '<')]">
<xsl:value-of select="FieldValue" disable-output-escaping="yes" />
</xsl:when>
Mark