diff --git a/src/schema_formats/HTML/HTML.xsl b/src/schema_formats/HTML/HTML.xsl
index 1065fdcb6..1a990aaa0 100644
--- a/src/schema_formats/HTML/HTML.xsl
+++ b/src/schema_formats/HTML/HTML.xsl
@@ -260,15 +260,29 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
     </xsl:choose>
   </xsl:template>
 
+  <xsl:template name="details-summary">
+    <xsl:param name="id"/>
+    <xsl:param name="text"/>
+    <summary id="{$id}"
+             style="margin-block-start: .83em; margin-block-end: .83em; cursor: pointer;">
+      <h2 style="display: inline;"><xsl:value-of select="$text"/></h2>
+    </summary>
+  </xsl:template>
+
   <!-- RNC preamble. -->
 
   <xsl:template name="rnc-preamble">
-    <h2 id="rnc_preamble">4 RNC Preamble</h2>
-    <div style="border: 1px solid; padding:10px; width: 85%; align: center; margin-left: auto; margin-right: auto; background: #d5d5d5;">
-      <pre>
-        <xsl:call-template name="preamble"/>
-      </pre>
-    </div>
+    <details open="">
+      <xsl:call-template name="details-summary">
+        <xsl:with-param name="id" select="'rnc_preamble'"/>
+        <xsl:with-param name="text" select="'4 RNC Preamble'"/>
+      </xsl:call-template>
+      <div style="border: 1px solid; padding:10px; width: 85%; align: center; margin-left: auto; margin-right: auto; background: #d5d5d5;">
+        <pre>
+          <xsl:call-template name="preamble"/>
+        </pre>
+      </div>
+    </details>
   </xsl:template>
 
   <!-- Types. -->
@@ -285,10 +299,15 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
   </xsl:template>
 
   <xsl:template name="type-summary">
-    <h2 id="type_summary">1 Summary of Data Types</h2>
-    <table id="index">
-    <xsl:apply-templates select="type" mode="index"/>
-    </table>
+    <details open="">
+      <xsl:call-template name="details-summary">
+        <xsl:with-param name="id" select="'type_summary'"/>
+        <xsl:with-param name="text" select="'1 Summary of Data Types'"/>
+      </xsl:call-template>
+      <table id="index">
+        <xsl:apply-templates select="type" mode="index"/>
+      </table>
+    </details>
   </xsl:template>
 
   <xsl:template match="type" mode="details">
@@ -325,8 +344,13 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
   </xsl:template>
 
   <xsl:template name="type-details">
-    <h2 id="type_details">5 Data Type Details</h2>
-    <xsl:apply-templates select="type" mode="details"/>
+    <details open="">
+      <xsl:call-template name="details-summary">
+        <xsl:with-param name="id" select="'type_details'"/>
+        <xsl:with-param name="text" select="'5 Data Type Details'"/>
+      </xsl:call-template>
+      <xsl:apply-templates select="type" mode="details"/>
+    </details>
   </xsl:template>
 
   <!-- Elements. -->
@@ -343,15 +367,25 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
   </xsl:template>
 
   <xsl:template name="element-summary">
-    <h2 id="element_summary">2 Summary of Elements</h2>
-    <table id="index">
-    <xsl:apply-templates select="element" mode="index"/>
-    </table>
+    <details open="">
+      <xsl:call-template name="details-summary">
+        <xsl:with-param name="id" select="'element_summary'"/>
+        <xsl:with-param name="text" select="'2 Summary of Elements'"/>
+      </xsl:call-template>
+      <table id="index">
+        <xsl:apply-templates select="element" mode="index"/>
+      </table>
+    </details>
   </xsl:template>
 
   <xsl:template name="element-details">
-    <h2 id="element_details">6 Element Details</h2>
-    <xsl:apply-templates select="element"/>
+    <details open="">
+      <xsl:call-template name="details-summary">
+        <xsl:with-param name="id" select="'element_details'"/>
+        <xsl:with-param name="text" select="'6 Element Details'"/>
+      </xsl:call-template>
+      <xsl:apply-templates select="element"/>
+    </details>
   </xsl:template>
 
   <xsl:template match="element">
@@ -691,15 +725,25 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
   </xsl:template>
 
   <xsl:template name="command-summary">
-    <h2 id="command_summary">3 Summary of Commands</h2>
-    <table id="index">
-    <xsl:apply-templates select="command" mode="index"/>
-    </table>
+    <details open="">
+      <xsl:call-template name="details-summary">
+        <xsl:with-param name="id" select="'command_summary'"/>
+        <xsl:with-param name="text" select="'3 Summary of Commands'"/>
+      </xsl:call-template>
+      <table id="index">
+        <xsl:apply-templates select="command" mode="index"/>
+      </table>
+    </details>
   </xsl:template>
 
   <xsl:template name="command-details">
-    <h2 id="command_details">7 Command Details</h2>
-    <xsl:apply-templates select="command"/>
+    <details open="">
+      <xsl:call-template name="details-summary">
+        <xsl:with-param name="id" select="'command_details'"/>
+        <xsl:with-param name="text" select="'7 Command Details'"/>
+      </xsl:call-template>
+      <xsl:apply-templates select="command"/>
+    </details>
   </xsl:template>
 
   <!-- Filter keywords -->
@@ -749,11 +793,16 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
   </xsl:template>
 
   <xsl:template name="changes">
-    <h2 id="changes">
-      8 Compatibility Changes in Version
-      <xsl:value-of select="/protocol/version"/>
-    </h2>
-    <xsl:apply-templates select="change[version=/protocol/version]"/>
+    <details open="">
+      <xsl:call-template name="details-summary">
+        <xsl:with-param name="id" select="'changes'"/>
+        <xsl:with-param name="text">
+          <xsl:value-of select="'8 Compatibility Changes in Version '"/>
+          <xsl:value-of select="/protocol/version"/>
+        </xsl:with-param>
+      </xsl:call-template>
+      <xsl:apply-templates select="change[version=/protocol/version]"/>
+    </details>
   </xsl:template>
 
   <!-- Deprecation Warnings. -->