doc/tools/base-html-stylesheet.xsl

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version='1.0'>

  <xsl:param name="html.stylesheet">styles.css</xsl:param>
  <xsl:param name="toc.section.depth">3</xsl:param>
  <xsl:param name="annotate.toc">0</xsl:param>

  <xsl:template match="sect1" mode="toc">
    <xsl:param name="toc-context" select="."/>
    <xsl:call-template name="subtoc">
      <xsl:with-param name="toc-context" select="$toc-context"/>
      <xsl:with-param name="nodes" 
        select="sect2|refentry|bridgehead[$bridgehead.in.toc != 0]"/>
    </xsl:call-template>
  </xsl:template>

  <xsl:template match="sect2" mode="toc">
    <xsl:param name="toc-context" select="."/>

    <xsl:call-template name="subtoc">
      <xsl:with-param name="toc-context" select="$toc-context"/>
      <xsl:with-param name="nodes" 
        select="sect3|refentry|bridgehead[$bridgehead.in.toc != 0]"/>
    </xsl:call-template>
  </xsl:template>

</xsl:stylesheet>