diff --git a/docs/manifest-format.md b/docs/manifest-format.md
index 8e5e287..6156333 100644
--- a/docs/manifest-format.md
+++ b/docs/manifest-format.md
@@ -109,6 +109,12 @@
 ]>
 ```
 
+For compatibility purposes across repo releases, all unknown elements are
+silently ignored.  However, repo reserves all possible names for itself for
+future use.  If you want to use custom elements, the `x-*` namespace is
+reserved for that purpose, and repo guarantees to never allocate any
+corresponding names.
+
 A description of the elements and their attributes follows.
 
 
diff --git a/tests/test_manifest_xml.py b/tests/test_manifest_xml.py
index e4adf3c..e2c83af 100644
--- a/tests/test_manifest_xml.py
+++ b/tests/test_manifest_xml.py
@@ -280,6 +280,27 @@
         '<superproject name="superproject"/>' +
         '</manifest>')
 
+  def test_unknown_tags(self):
+    """Check superproject settings."""
+    manifest = self.getXmlManifest("""
+<manifest>
+  <remote name="test-remote" fetch="http://localhost" />
+  <default remote="test-remote" revision="refs/heads/main" />
+  <superproject name="superproject"/>
+  <iankaz value="unknown (possible) future tags are ignored"/>
+  <x-custom-tag>X tags are always ignored</x-custom-tag>
+</manifest>
+""")
+    self.assertEqual(manifest.superproject['name'], 'superproject')
+    self.assertEqual(manifest.superproject['remote'].name, 'test-remote')
+    self.assertEqual(
+        manifest.ToXml().toxml(),
+        '<?xml version="1.0" ?><manifest>' +
+        '<remote name="test-remote" fetch="http://localhost"/>' +
+        '<default remote="test-remote" revision="refs/heads/main"/>' +
+        '<superproject name="superproject"/>' +
+        '</manifest>')
+
   def test_project_group(self):
     """Check project group settings."""
     manifest = self.getXmlManifest("""