diff --git a/testing/web-platform/meta/MANIFEST.json b/testing/web-platform/meta/MANIFEST.json
index 67bbc982a67f..c45746b8a863 100644
--- a/testing/web-platform/meta/MANIFEST.json
+++ b/testing/web-platform/meta/MANIFEST.json
@@ -542622,11 +542622,11 @@
    "testharness"
   ],
   "custom-elements/Document-createElement.html": [
-   "07de690760040f025455b1588ea00e57c669795f",
+   "46e64c9f412fb04582f8ec287e08783ac83cb933",
    "testharness"
   ],
   "custom-elements/Document-createElementNS.html": [
-   "57791273d1721711dcb456fe7409a734d37278a3",
+   "da90b2a1c13cf18fd5cade85dcae2dadef6243c9",
    "testharness"
   ],
   "custom-elements/HTMLElement-constructor.html": [
@@ -569338,7 +569338,7 @@
    "support"
   ],
   "interfaces/dom.idl": [
-   "ff96179e520ad069ec36a0ced2a6f2ceb2e8c5da",
+   "2d65a8029a80cfc97398d18f911f1ae65067e765",
    "support"
   ],
   "interfaces/fullscreen.idl": [
diff --git a/testing/web-platform/tests/custom-elements/Document-createElement.html b/testing/web-platform/tests/custom-elements/Document-createElement.html
index 21f024c5735b..ad29b91d88da 100644
--- a/testing/web-platform/tests/custom-elements/Document-createElement.html
+++ b/testing/web-platform/tests/custom-elements/Document-createElement.html
@@ -50,6 +50,21 @@ test(function () {
     assert_true(instance2 instanceof DefinedLater);
 }, 'document.createElement must create an instance of autonomous custom elements when it has is attribute');
 
+test(() => {
+    class SuperP extends HTMLParagraphElement {}
+    customElements.define("super-p", SuperP, { extends: "p" });
+
+    const superP = document.createElement("p", { is: "super-p" });
+    assert_true(superP instanceof HTMLParagraphElement);
+    assert_true(superP instanceof SuperP);
+    assert_equals(superP.localName, "p");
+
+    const notSuperP = document.createElement("p", "super-p");
+    assert_true(notSuperP instanceof HTMLParagraphElement);
+    assert_false(notSuperP instanceof SuperP);
+    assert_equals(notSuperP.localName, "p");
+}, "document.createElement()'s second argument is to be ignored when it's a string");
+
 function assert_reports(expected, testFunction, message) {
     var uncaughtError = null;
     window.onerror = function (message, url, lineNumber, columnNumber, error) { uncaughtError = error; return true; }
diff --git a/testing/web-platform/tests/custom-elements/Document-createElementNS.html b/testing/web-platform/tests/custom-elements/Document-createElementNS.html
index 4cc4561f54cf..6377ba63fca1 100644
--- a/testing/web-platform/tests/custom-elements/Document-createElementNS.html
+++ b/testing/web-platform/tests/custom-elements/Document-createElementNS.html
@@ -49,5 +49,20 @@ test(() => {
   assert_false(element instanceof MyBuiltinElement2);
   assert_false(element.hasAttribute('is'));
 }, 'builtin: document.createElementNS should check namespaces.');
+
+test(() => {
+    class SuperP extends HTMLParagraphElement {}
+    customElements.define("super-p", SuperP, { extends: "p" });
+
+    const superP = document.createElementNS("http://www.w3.org/1999/xhtml", "p", { is: "super-p" });
+    assert_true(superP instanceof HTMLParagraphElement);
+    assert_true(superP instanceof SuperP);
+    assert_equals(superP.localName, "p");
+
+    const notSuperP = document.createElementNS("http://www.w3.org/1999/xhtml", "p", "super-p");
+    assert_true(notSuperP instanceof HTMLParagraphElement);
+    assert_false(notSuperP instanceof SuperP);
+    assert_equals(notSuperP.localName, "p");
+}, "document.createElementNS()'s third argument is to be ignored when it's a string");
 </script>
 </body>
diff --git a/testing/web-platform/tests/interfaces/dom.idl b/testing/web-platform/tests/interfaces/dom.idl
index 5582fc55acfc..ae65018c71b8 100644
--- a/testing/web-platform/tests/interfaces/dom.idl
+++ b/testing/web-platform/tests/interfaces/dom.idl
@@ -266,8 +266,8 @@ interface Document : Node {
   HTMLCollection getElementsByTagNameNS(DOMString? namespace, DOMString localName);
   HTMLCollection getElementsByClassName(DOMString classNames);
 
-  [NewObject] Element createElement(DOMString localName, optional ElementCreationOptions options);
-  [NewObject] Element createElementNS(DOMString? namespace, DOMString qualifiedName, optional ElementCreationOptions options);
+  [NewObject] Element createElement(DOMString localName, optional (DOMString or ElementCreationOptions) options);
+  [NewObject] Element createElementNS(DOMString? namespace, DOMString qualifiedName, optional (DOMString or ElementCreationOptions) options);
   [NewObject] DocumentFragment createDocumentFragment();
   [NewObject] Text createTextNode(DOMString data);
   [NewObject] CDATASection createCDATASection(DOMString data);