You could use
<a>
<xsl:attribute name="href">
<xsl:value-of select="xxxx"/>
<xsl:value-of select="yyyy"/>
</xsl:attribute>
or
<a href="{xxxx}{yyyy}">
or
<a href="{concat(xxxx, yyyy)}">
or
<a>
<xsl:attribute name="href">
<xsl:value-of select="concat(xxxx, yyyy)"/
</xsl:attribute>
I cana't help you with xxxx and yyyy because you haven't said anything about your source document (you've just shown 50 lines of stylesheet code which don't seem to have any obvious relevance to your question)
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference