From c61c544d732fb818d872313b9124c7cd5f375d46 Mon Sep 17 00:00:00 2001
From: Norman Walsh <ndw@nwalsh.com>
Date: Tue, 3 Dec 2024 17:47:54 +0000
Subject: [PATCH] Support different options for dealing with unhandled elements

---
 src/guide/xml/ref-modes.xml         | 22 ++++++++
 src/guide/xml/ref-params.xml        | 49 +++++++++++++++++
 src/main/scss/media-all.scss        | 13 ++++-
 src/main/xslt/modules/unhandled.xsl | 84 +++++++++++++----------------
 4 files changed, 120 insertions(+), 48 deletions(-)

diff --git a/src/guide/xml/ref-modes.xml b/src/guide/xml/ref-modes.xml
index a0e5054f78..7ca640d33e 100644
--- a/src/guide/xml/ref-modes.xml
+++ b/src/guide/xml/ref-modes.xml
@@ -2030,5 +2030,27 @@ Be aware, however, that the HTML markup is quite constrained for panel sets.
 </refsection>
 </refentry>
 
+<refentry xml:id="m_unhandled">
+<?db filename="m_unhandled"?>
+<refmeta>
+<refentrytitle>m:unhandled</refentrytitle>
+<refmiscinfo>{http://docbook.org/ns/docbook/modes}unhandled</refmiscinfo>
+<refmiscinfo class="version">2.5.1</refmiscinfo>
+</refmeta>
+<refnamediv>
+<refname>m:unhandled</refname>
+<refpurpose>Process unhandled elements</refpurpose>
+<refclass>mode</refclass>
+</refnamediv>
+<refsection>
+<title>Description</title>
+<para>When <parameter>on-unhandled-elements</parameter> is set to <literal>custom</literal>,
+elements with no other matching template will be processed in this mode. Note that
+there are no templates in this mode in the standard stylesheets. This is an extensibility
+point for customization layers.
+</para>
+</refsection>
+</refentry>
+
 </reference>
 
diff --git a/src/guide/xml/ref-params.xml b/src/guide/xml/ref-params.xml
index ca8e3561f8..626087b6f5 100644
--- a/src/guide/xml/ref-params.xml
+++ b/src/guide/xml/ref-params.xml
@@ -5898,4 +5898,53 @@ unless a processing instruction overrides it.</para>
 </refsection>
 </refentry>  
 
+<refentry>
+  <refmeta>
+    <fieldsynopsis>
+      <type>xs:string</type>
+      <varname>on-unhandled-elements</varname>
+      <initializer>'fail'</initializer>
+    </fieldsynopsis>
+    <refmiscinfo class="version">2.5.1</refmiscinfo>
+  </refmeta>
+  <refnamediv>
+    <refpurpose>How to process unhandled elements</refpurpose>
+  </refnamediv>
+<refsection>
+<title>Description</title>
+
+<para>This parameter controls how unhandled elements (for example, extension
+elements added in a custom schema) should be processed.</para>
+
+<variablelist role="enumeration">
+<varlistentry>
+<term>
+<enumvalue>terminate</enumvalue>
+</term>
+<listitem>
+<para>Generates an error messages and aborts the transformation.</para>
+</listitem>
+</varlistentry>
+<varlistentry>
+<term>
+<enumvalue>render</enumvalue>
+</term>
+<listitem>
+<para>Renders the tag name in the output, styled in red and yellow.</para>
+</listitem>
+</varlistentry>
+<varlistentry>
+<term>
+<enumvalue>custom</enumvalue>
+</term>
+<listitem>
+<para>Processes the template in the <mode>m:unhandled</mode> mode. These must be defined
+in a customization layer; there are no templates in this mode in the standard stylesheets.
+</para>
+</listitem>
+</varlistentry>
+</variablelist>
+</refsection>
+</refentry>  
+
 </reference>
diff --git a/src/main/scss/media-all.scss b/src/main/scss/media-all.scss
index 56f0178190..432c5b36de 100644
--- a/src/main/scss/media-all.scss
+++ b/src/main/scss/media-all.scss
@@ -12,7 +12,7 @@
     --on-primary-color: #ffffff;
     --on-primary-variant-color: #ffffff;
     --on-secondary-color: #000000;
-    --on-error-color: #000000;
+    --on-error-color: #ffee42;
     --enabled-color: rgb(240,240,240);
     --hovered-color: rgb(235,235,235);
     --focused-color: rgb(221,221,221);
@@ -201,6 +201,17 @@ span.error.broken-link::after {
     content: " 🔗 ";
 }
 
+.unhandled {
+    font-family: monospace;
+    background-color: var(--error-border-color);
+    color: var(--on-error-color);
+    font-weight: bold;
+}
+
+div.unhandled {
+    width: 100%;
+}
+
 .firstterm {
     font-weight: bold;
 }
diff --git a/src/main/xslt/modules/unhandled.xsl b/src/main/xslt/modules/unhandled.xsl
index 333b1f2adf..6efae7790b 100644
--- a/src/main/xslt/modules/unhandled.xsl
+++ b/src/main/xslt/modules/unhandled.xsl
@@ -9,53 +9,43 @@
                 version="3.0">
 
 <xsl:template match="*">
-  <xsl:message terminate="yes"
-      select="'No template for ' || node-name(.) || ': ' || f:generate-id(.)"/>
-  <xsl:variable name="inline-style"
-                select="'font: monospace;
-                         color: yellow;
-                         background-color: red;
-                         font-weight: bold;'"/>
-  <xsl:variable name="block-style"
-                select="'font: monospace;
-                         color: yellow;
-                         width: 100%;
-                         background-color: red;
-                         font-weight: bold;'"/>
-
-  <xsl:variable name="inline"
-                select="normalize-space(string-join(text(),'')) != ''"/>
-
-  <xsl:element namespace="http://www.w3.org/1999/xhtml"
-               name="{if ($inline) then 'span' else 'div'}">
-
-    <xsl:element namespace="http://www.w3.org/1999/xhtml"
-                 name="{if ($inline) then 'span' else 'div'}">
-      <xsl:attribute name="style"
-                     select="if ($inline) then $inline-style else $block-style"/>
-      <xsl:text>&lt;</xsl:text>
-      <xsl:value-of select="node-name(.)"/>
-      <xsl:text>&gt;</xsl:text>
-    </xsl:element>
-
-    <xsl:apply-templates/>
-
-    <xsl:element namespace="http://www.w3.org/1999/xhtml"
-                 name="{if ($inline) then 'span' else 'div'}">
-      <xsl:attribute name="style"
-                     select="if ($inline) then $inline-style else $block-style"/>
-      <xsl:text>&lt;/</xsl:text>
-      <xsl:value-of select="node-name(.)"/>
-      <xsl:text>&gt;</xsl:text>
-    </xsl:element>
-  </xsl:element>
-
-  <!--
-    <xsl:message>
-      <xsl:text>Unhandled element: </xsl:text>
-      <xsl:value-of select="node-name(.)"/>
-    </xsl:message>
-  -->
+  <xsl:choose>
+    <xsl:when test="$on-unhandled-elements = 'fail'">
+      <xsl:message terminate="yes"
+                   select="'No template for ' || node-name(.) || ': ' || f:generate-id(.)"/>
+    </xsl:when>
+    <xsl:when test="$on-unhandled-elements = 'render'">
+      <xsl:message select="'No template for ' || node-name(.) || ': ' || f:generate-id(.)"/>
+
+      <xsl:variable name="inline"
+                    select="normalize-space(string-join(text(),'')) != ''"/>
+
+      <xsl:element namespace="http://www.w3.org/1999/xhtml"
+                   name="{if ($inline) then 'span' else 'div'}">
+        
+        <xsl:element namespace="http://www.w3.org/1999/xhtml"
+                     name="{if ($inline) then 'span' else 'div'}">
+          <xsl:attribute name="class" select="'unhandled'"/>
+          <xsl:text>&lt;</xsl:text>
+          <xsl:value-of select="node-name(.)"/>
+          <xsl:text>&gt;</xsl:text>
+        </xsl:element>
+
+        <xsl:apply-templates/>
+
+        <xsl:element namespace="http://www.w3.org/1999/xhtml"
+                     name="{if ($inline) then 'span' else 'div'}">
+          <xsl:attribute name="class" select="'unhandled'"/>
+          <xsl:text>&lt;/</xsl:text>
+          <xsl:value-of select="node-name(.)"/>
+          <xsl:text>&gt;</xsl:text>
+        </xsl:element>
+      </xsl:element>
+    </xsl:when>
+    <xsl:otherwise>
+      <xsl:apply-templates select="." mode="m:unhandled"/>
+    </xsl:otherwise>
+  </xsl:choose>
 </xsl:template>
 
 <xsl:template xmlns:h="http://www.w3.org/1999/xhtml"