<xsl:stylesheet extension-element-prefixes="saxon" version="1.0">
   <xsl:output method="xml" indent="yes"/>

   <xsl:template match="//AAA">
     <html>
       <body>
         <xsl:apply-templates select="BBB|//CCC">
           <xsl:with-param name="par_a">ZZ</xsl:with-param>
         </xsl:apply-templates>
       </body>
     </html>
   </xsl:template>

   <xsl:template match="BBB">
     <xsl:param name="par_a"/>
     <h2>
       <xsl:value-of select="$par_a"/>
       <xsl:value-of select="text()"/>
     </h2>
   </xsl:template>

   <xsl:template match="CCC">
     <p>
       <xsl:value-of select="text()"/>
     </p>
   </xsl:template>
 </xsl:stylesheet>