Index: trunk/src/org/openstreetmap/josm/io/ValidatorErrorWriter.java
===================================================================
--- trunk/src/org/openstreetmap/josm/io/ValidatorErrorWriter.java	(revision 17988)
+++ trunk/src/org/openstreetmap/josm/io/ValidatorErrorWriter.java	(revision 17989)
@@ -11,10 +11,11 @@
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Comparator;
 import java.util.HashMap;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Set;
-import java.util.TreeSet;
 import java.util.stream.Collectors;
 
@@ -63,5 +64,6 @@
      */
     public void write(Collection<TestError> validationErrors) throws IOException {
-        Set<Test> analysers = validationErrors.stream().map(TestError::getTester).collect(Collectors.toCollection(TreeSet::new));
+        Set<Test> analysers = validationErrors.stream().map(TestError::getTester)
+                .sorted(Comparator.comparing(t -> t.getSource().toString())).collect(Collectors.toCollection(LinkedHashSet::new));
         String timestamp = Instant.now().toString();
 
@@ -100,4 +102,5 @@
                         out.println("/>");
                         for (OsmPrimitive p : error.getPrimitives()) {
+                            out.print("    ");
                             p.accept(osmWriter);
                         }
@@ -126,8 +129,8 @@
                 out.println("  </analyser>");
             }
+
+            out.println("</analysers>");
+            out.flush();
         }
-
-        out.println("</analysers>");
-        out.flush();
     }
 
Index: trunk/test/unit/org/openstreetmap/josm/io/ValidatorErrorWriterTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/io/ValidatorErrorWriterTest.java	(revision 17989)
+++ trunk/test/unit/org/openstreetmap/josm/io/ValidatorErrorWriterTest.java	(revision 17989)
@@ -0,0 +1,70 @@
+// License: GPL. For details, see LICENSE file.
+package org.openstreetmap.josm.io;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+import org.openstreetmap.josm.data.coor.LatLon;
+import org.openstreetmap.josm.data.osm.Node;
+import org.openstreetmap.josm.data.validation.Severity;
+import org.openstreetmap.josm.data.validation.TestError;
+import org.openstreetmap.josm.data.validation.tests.RightAngleBuildingTest;
+import org.openstreetmap.josm.testutils.JOSMTestRules;
+
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+
+/**
+ * Unit tests of {@link ValidatorErrorWriter}
+ */
+class ValidatorErrorWriterTest {
+
+    /**
+     * Setup rule
+     */
+    @RegisterExtension
+    @SuppressFBWarnings(value = "URF_UNREAD_PUBLIC_OR_PROTECTED_FIELD")
+    public JOSMTestRules test = new JOSMTestRules().preferences();
+
+    @Test
+    void testEmpty() throws IOException {
+        doTest(Collections.emptyList(), "");
+    }
+
+    @Test
+    void testErrors() throws IOException {
+        doTest(Arrays.asList(TestError.builder(new RightAngleBuildingTest(), Severity.OTHER, 3701)
+                .message("Building with an almost square angle")
+                .primitives(new Node(LatLon.NORTH_POLE)).build()),
+                  "  <analyser timestamp='\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}\\.\\d{3}Z' name='Almost right angle buildings'>"
+                + "    <class id='1' level='3'>"
+                + "      <classtext lang='fr' title='Building with an almost square angle'/>"
+                + "    </class>"
+                + "    <error class='1'>"
+                + "      <location lat='90\\.0' lon='0\\.0'/>"
+                + "      <node id='-1' visible='true' lat='90\\.0' lon='0\\.0' />"
+                + "      <text lang='fr' value='null'/>"
+                + "    </error>"
+                + "  </analyser>");
+    }
+
+    private static void doTest(Collection<TestError> validationErrors, String content) throws IOException {
+        ByteArrayOutputStream baos = new ByteArrayOutputStream();
+        try (ValidatorErrorWriter writer = new ValidatorErrorWriter(baos)) {
+            writer.write(validationErrors);
+        }
+        String xml = new String(baos.toByteArray(), StandardCharsets.UTF_8);
+        assertTrue(xml.trim().replace("\r", "").replace("\n", "")
+                .matches("<\\?xml version='1.0' encoding='UTF-8'\\?>" +
+                        "<analysers generator='JOSM' timestamp='\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}\\.\\d{3}Z'>" +
+                        content +
+                        "</analysers>"), xml);
+    }
+}