diff --git a/custom-elements/Document-createElement.html b/custom-elements/Document-createElement.html
index 21f024c5735bae..ad29b91d88da8c 100644
--- a/custom-elements/Document-createElement.html
+++ b/custom-elements/Document-createElement.html
@@ -50,6 +50,21 @@
     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/custom-elements/Document-createElementNS.html b/custom-elements/Document-createElementNS.html
index 4cc4561f54cf59..6377ba63fca15a 100644
--- a/custom-elements/Document-createElementNS.html
+++ b/custom-elements/Document-createElementNS.html
@@ -49,5 +49,20 @@
   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/interfaces/dom.idl b/interfaces/dom.idl
index 90419f083f8959..e97f803f82105e 100644
--- a/interfaces/dom.idl
+++ b/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);