Issue #6520 - Fixing ErrorHandler output of text/html

+ Updating tests to ensure that output is xml verified
+ Updating output to use `<hr>` element properly.

Signed-off-by: Joakim Erdfelt <joakim.erdfelt@gmail.com>

jetty-server/src/main/java/org/eclipse/jetty/server/handler/ErrorHandler.java

@@ -397,7 +397,7 @@ protected void writeErrorPageBody(HttpServletRequest request, Writer writer, int
             writeErrorPageStacks(request, writer);
 
         Request.getBaseRequest(request).getHttpChannel().getHttpConfiguration()
-            .writePoweredBy(writer, "<hr>", "<hr/>\n");
+            .writePoweredBy(writer, "<hr/>", "<hr/>\n");
     }
 
     protected void writeErrorPageMessage(HttpServletRequest request, Writer writer, int code, String message, String uri)

jetty-server/src/test/java/org/eclipse/jetty/server/ErrorHandlerTest.java

@@ -18,7 +18,9 @@
 
 package org.eclipse.jetty.server;
 
+import java.io.ByteArrayInputStream;
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
@@ -27,6 +29,9 @@
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
+import javax.xml.XMLConstants;
+import javax.xml.parsers.DocumentBuilder;
+import javax.xml.parsers.DocumentBuilderFactory;
 
 import org.eclipse.jetty.http.BadMessageException;
 import org.eclipse.jetty.http.HttpHeader;
@@ -43,6 +48,7 @@
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.ValueSource;
+import org.w3c.dom.Document;
 
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.anyOf;
@@ -567,7 +573,7 @@ public void testComplexCauseMessageAcceptUtf8Header(String path) throws Exceptio
         }
     }
 
-    private String assertContent(HttpTester.Response response)
+    private String assertContent(HttpTester.Response response) throws Exception
     {
         String contentType = response.get(HttpHeader.CONTENT_TYPE);
         String content = response.getContent();
@@ -578,6 +584,17 @@ private String assertContent(HttpTester.Response response)
             assertThat(content, not(containsString("<glossary>")));
             assertThat(content, not(containsString("<!DOCTYPE>")));
             assertThat(content, not(containsString("&euro;")));
+
+            // we expect that our generated output conforms to text/xhtml is well formed
+            DocumentBuilderFactory xmlDocumentBuilderFactory = DocumentBuilderFactory.newInstance();
+            xmlDocumentBuilderFactory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);
+            DocumentBuilder db = xmlDocumentBuilderFactory.newDocumentBuilder();
+            try (ByteArrayInputStream inputStream = new ByteArrayInputStream(content.getBytes(StandardCharsets.UTF_8)))
+            {
+                // We consider this content to be XML well formed if these 2 lines do not throw an Exception
+                Document doc = db.parse(inputStream);
+                doc.getDocumentElement().normalize();
+            }
         }
         else if (contentType.contains("text/json"))
         {