diff --git a/build.xml b/build.xml
index 6f7fc08c3..6ea84ef73 100644
--- a/build.xml
+++ b/build.xml
@@ -1503,6 +1503,14 @@
         <patternset refid="useful.tests"/>
       </srcfiles>
     </uptodate>
+    <!-- Javadoc on classes in Ant project, don't (yet) comply to HTML5
+    semantics. javadoc tool in JDK 9+ allows specifying the HTML version
+    to use for the output generation. We intentionally set it to -html4
+    to prevent javadoc tool failures (in JDK 11, which is strict) when
+    running against our code -->
+    <condition property="javadoc.output.htmlversion" value="-html4" else="">
+      <isset property="jdk9+"/>
+    </condition>
   </target>
 
   <target name="javadocs" depends="check-javadoc"
@@ -1535,6 +1543,7 @@
       <group title="Optional Types" packages="org.apache.tools.ant.types.optional*"/>
       <group title="Ant Utilities" packages="org.apache.tools.ant.util*"/>
       <classpath refid="tests-classpath"/>
+      <arg line="${javadoc.output.htmlversion}"/>
     </javadoc>
   </target>