--- a/testing/web-platform/meta/MANIFEST.json
+++ b/testing/web-platform/meta/MANIFEST.json
@@ -537414,17 +537414,17 @@
    "9af8f2dc7778feeea4fa8e557d7885b10d325dea",
    "testharness"
   ],
   "custom-elements/Document-createElement.html": [
    "2275720296bd3ea4ef012ed5ef3ea63a4c5f953d",
    "testharness"
   ],
   "custom-elements/Document-createElementNS.html": [
-   "04cd97839e98c6082f67740dbcfc7588e2b251b9",
+   "799f59e3bf8ab0830e44faa3ffef6d3303da42eb",
    "testharness"
   ],
   "custom-elements/HTMLElement-constructor.html": [
    "64522527ef425b90c704b20b000c8feef0d1ca25",
    "testharness"
   ],
   "custom-elements/OWNERS": [
    "9f6553b67cad3b479d3beb678653db4e712ed227",

--- a/testing/web-platform/tests/custom-elements/Document-createElementNS.html
+++ b/testing/web-platform/tests/custom-elements/Document-createElementNS.html
@@ -19,16 +19,24 @@ test(() => {
   class MyElement2 extends HTMLElement {};
 
   customElements.define('my-autonomous2', MyElement2);
   let element = document.createElementNS('urn:example', 'my-autonomous2');
   assert_false(element instanceof MyElement2);
 }, 'autonomous: document.createElementNS should check namespaces.');
 
 test(() => {
+  const xhtmlNS = 'http://www.w3.org/1999/xhtml';
+  assert_false(document.createElementNS(xhtmlNS, 'x-foo') instanceof HTMLUnknownElement);
+  assert_false(document.createElementNS(xhtmlNS, 'x-foo', {}) instanceof HTMLUnknownElement);
+  assert_false((new Document()).createElementNS(xhtmlNS, 'x-foo') instanceof HTMLUnknownElement);
+  assert_false((new Document()).createElementNS(xhtmlNS, 'x-foo', {}) instanceof HTMLUnknownElement);
+}, 'autonomous: document.createElementNS should not create HTMLUnknownElement for a valid custom element name');
+
+test(() => {
   class MyBuiltinElement extends HTMLElement {};
 
   customElements.define('my-builtin', MyBuiltinElement, { extends: 'address' });
   let element = document.createElementNS('http://www.w3.org/1999/xhtml', 'p:address', { is: 'my-builtin'});
   assert_true(element instanceof MyBuiltinElement);
   assert_equals(element.prefix, 'p');
 }, 'builtin: document.createElementNS should create custom elements with prefixes.');