Very rough XSLT to backlink from DocBook's glossary to the first glossterm|firstterm entry

glossary.xml

<article xml:lang="en" version="5.0"
  xmlns="http://docbook.org/ns/docbook"
  xmlns:xi="http://www.w3.org/2001/XInclude"
  xmlns:xlink="http://www.w3.org/1999/xlink">
    <title>Test</title>
    <para xml:id="first"><glossterm>XML</glossterm></para>
    <para xml:id="second">bla bla bla <glossterm>XML</glossterm></para>
  
    <glossary>
      <title>Glossary</title>
      <glossentry xml:id="glos.xml">
        <glossterm>XML</glossterm>
        <glossdef>
          <para>Definition of XML</para>
        </glossdef>
      </glossentry>
    </glossary>
</article>

glossarybacklink_html.xsl

<xsl:stylesheet version="1.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns="http://www.w3.org/1999/xhtml"
  xmlns:d="http://docbook.org/ns/docbook">
  
  <!-- This is our key; we are interested in glossterms and firstterms elements
       and its (text) contents
  -->
  <xsl:key name="glossterm" match="d:glossterm|d:firstterm" use="."/>
  
  <xsl:template match="d:article|d:glossary|d:glossentry">
    <div class="{local-name()}">
      <xsl:if test="@xml:id">
        <xsl:attribute name="id"><xsl:value-of select="@xml:id"/></xsl:attribute>
      </xsl:if>
      <xsl:apply-templates/>
    </div>
  </xsl:template>
  
  <xsl:template match="d:glossentry/d:glossdef">
    <xsl:param name="node"/>
    <xsl:variable name="term" select="../d:glossterm"/>
    <!-- Select the target of the first glossterm or firstterm pointing to
         this glossterm definition
         
         From all found targets, use only the first one.
    -->
    <xsl:variable name="target" select="key('glossterm', $term)[1]"/>
    
    <!-- Just some debug message -->
    <xsl:message>d:glossentry/d:glossdef
      glossterm=<xsl:value-of select="$term"/>
      target=<xsl:value-of select="count($target)"/>
    </xsl:message>
    
    <xsl:if test="$target">
      <!-- Here we use only the a message, however, it can be created anything.
           ATM we use the ID from its parent element. This can be improved by
           using the ancestor-or-self axis:
           
           $target/ancestor-or-self::*/@xml:id
      -->
      <xsl:message>glossterm with <xsl:value-of select="$target/../@xml:id"/></xsl:message>
    </xsl:if>
  </xsl:template>
  
</xsl:stylesheet>