--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -5654,45 +5654,50 @@ nsDocument::CreateElement(const nsAStrin
 }
 
 NS_IMETHODIMP
 nsDocument::CreateElementNS(const nsAString& aNamespaceURI,
                             const nsAString& aQualifiedName,
                             nsIDOMElement** aReturn)
 {
   *aReturn = nullptr;
-  ElementCreationOptions options;
+  ElementCreationOptionsOrString options;
+
+  options.SetAsString();
   ErrorResult rv;
   nsCOMPtr<Element> element =
     CreateElementNS(aNamespaceURI, aQualifiedName, options, rv);
   NS_ENSURE_FALSE(rv.Failed(), rv.StealNSResult());
   return CallQueryInterface(element, aReturn);
 }
 
 already_AddRefed<Element>
 nsDocument::CreateElementNS(const nsAString& aNamespaceURI,
                             const nsAString& aQualifiedName,
-                            const ElementCreationOptions& aOptions,
+                            const ElementCreationOptionsOrString& aOptions,
                             ErrorResult& rv)
 {
   RefPtr<mozilla::dom::NodeInfo> nodeInfo;
   rv = nsContentUtils::GetNodeInfoFromQName(aNamespaceURI,
                                             aQualifiedName,
                                             mNodeInfoManager,
                                             nsIDOMNode::ELEMENT_NODE,
                                             getter_AddRefs(nodeInfo));
   if (rv.Failed()) {
     return nullptr;
   }
 
-  // Throw NotFoundError if 'is' is not-null and definition is null
-  const nsString* is = CheckCustomElementName(
-    aOptions, aQualifiedName, nodeInfo->NamespaceID(), rv);
-  if (rv.Failed()) {
-    return nullptr;
+  const nsString* is = nullptr;
+  if (aOptions.IsElementCreationOptions()) {
+    // Throw NotFoundError if 'is' is not-null and definition is null
+    is = CheckCustomElementName(aOptions.GetAsElementCreationOptions(),
+                                aQualifiedName, nodeInfo->NamespaceID(), rv);
+    if (rv.Failed()) {
+      return nullptr;
+    }
   }
 
   nsCOMPtr<Element> element;
   rv = NS_NewElement(getter_AddRefs(element), nodeInfo.forget(),
                      NOT_FROM_PARSER, is);
   if (rv.Failed()) {
     return nullptr;
   }

--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -1135,17 +1135,17 @@ public:
   virtual void GetLastStyleSheetSet(nsString& aSheetSet) override;
   virtual mozilla::dom::DOMStringList* StyleSheetSets() override;
   virtual void EnableStyleSheetsForSet(const nsAString& aSheetSet) override;
   virtual already_AddRefed<Element> CreateElement(const nsAString& aTagName,
                                                   const mozilla::dom::ElementCreationOptionsOrString& aOptions,
                                                   ErrorResult& rv) override;
   virtual already_AddRefed<Element> CreateElementNS(const nsAString& aNamespaceURI,
                                                     const nsAString& aQualifiedName,
-                                                    const mozilla::dom::ElementCreationOptions& aOptions,
+                                                    const mozilla::dom::ElementCreationOptionsOrString& aOptions,
                                                     mozilla::ErrorResult& rv) override;
 
   virtual nsIDocument* MasterDocument() override
   {
     return mMasterDocument ? mMasterDocument.get()
                            : this;
   }
 

--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -2603,17 +2603,17 @@ public:
   // GetElementById defined above
   virtual already_AddRefed<Element>
     CreateElement(const nsAString& aTagName,
                   const mozilla::dom::ElementCreationOptionsOrString& aOptions,
                   mozilla::ErrorResult& rv) = 0;
   virtual already_AddRefed<Element>
     CreateElementNS(const nsAString& aNamespaceURI,
                     const nsAString& aQualifiedName,
-                    const mozilla::dom::ElementCreationOptions& aOptions,
+                    const mozilla::dom::ElementCreationOptionsOrString& aOptions,
                     mozilla::ErrorResult& rv) = 0;
   already_AddRefed<mozilla::dom::DocumentFragment>
     CreateDocumentFragment() const;
   already_AddRefed<nsTextNode> CreateTextNode(const nsAString& aData) const;
   already_AddRefed<mozilla::dom::Comment>
     CreateComment(const nsAString& aData) const;
   already_AddRefed<mozilla::dom::ProcessingInstruction>
     CreateProcessingInstruction(const nsAString& target, const nsAString& data,

--- a/dom/webidl/Document.webidl
+++ b/dom/webidl/Document.webidl
@@ -50,17 +50,17 @@ interface Document : Node {
   [Pure]
   HTMLCollection getElementsByClassName(DOMString classNames);
   [Pure]
   Element? getElementById(DOMString elementId);
 
   [NewObject, Throws]
   Element createElement(DOMString localName, optional (ElementCreationOptions or DOMString) options);
   [NewObject, Throws]
-  Element createElementNS(DOMString? namespace, DOMString qualifiedName, optional ElementCreationOptions options);
+  Element createElementNS(DOMString? namespace, DOMString qualifiedName, optional (ElementCreationOptions or DOMString) options);
   [NewObject]
   DocumentFragment createDocumentFragment();
   [NewObject]
   Text createTextNode(DOMString data);
   [NewObject]
   Comment createComment(DOMString data);
   [NewObject, Throws]
   ProcessingInstruction createProcessingInstruction(DOMString target, DOMString data);

--- a/dom/xml/XMLDocument.cpp
+++ b/dom/xml/XMLDocument.cpp
@@ -159,19 +159,21 @@ NS_NewDOMDocument(nsIDOMDocument** aInst
     d->AppendChild(*doctypeAsNode, result);
     if (NS_WARN_IF(result.Failed())) {
       return result.StealNSResult();
     }
   }
 
   if (!aQualifiedName.IsEmpty()) {
     ErrorResult result;
+    ElementCreationOptionsOrString options;
+    options.SetAsString();
+
     nsCOMPtr<Element> root =
-      doc->CreateElementNS(aNamespaceURI, aQualifiedName,
-                           ElementCreationOptions(), result);
+      doc->CreateElementNS(aNamespaceURI, aQualifiedName, options, result);
     if (NS_WARN_IF(result.Failed())) {
       return result.StealNSResult();
     }
 
     d->AppendChild(*root, result);
     if (NS_WARN_IF(result.Failed())) {
       return result.StealNSResult();
     }

--- a/dom/xslt/xslt/txMozillaXSLTProcessor.cpp
+++ b/dom/xslt/xslt/txMozillaXSLTProcessor.cpp
@@ -1121,19 +1121,22 @@ txMozillaXSLTProcessor::notifyError()
     MOZ_ASSERT(document->GetReadyStateEnum() ==
                  nsIDocument::READYSTATE_UNINITIALIZED,
                "Bad readyState.");
     document->SetReadyStateInternal(nsIDocument::READYSTATE_LOADING);
 
     NS_NAMED_LITERAL_STRING(ns, "http://www.mozilla.org/newlayout/xml/parsererror.xml");
 
     IgnoredErrorResult rv;
+    ElementCreationOptionsOrString options;
+    options.SetAsString();
+
     nsCOMPtr<Element> element =
         document->CreateElementNS(ns, NS_LITERAL_STRING("parsererror"),
-                                  ElementCreationOptions(), rv);
+                                  options, rv);
     if (rv.Failed()) {
         return;
     }
 
     document->AppendChild(*element, rv);
     if (rv.Failed()) {
         return;
     }
@@ -1141,19 +1144,22 @@ txMozillaXSLTProcessor::notifyError()
     RefPtr<nsTextNode> text = document->CreateTextNode(mErrorText);
 
     element->AppendChild(*text, rv);
     if (rv.Failed()) {
         return;
     }
 
     if (!mSourceText.IsEmpty()) {
+        ElementCreationOptionsOrString options;
+        options.SetAsString();
+
         nsCOMPtr<Element> sourceElement =
             document->CreateElementNS(ns, NS_LITERAL_STRING("sourcetext"),
-                                      ElementCreationOptions(), rv);
+                                      options, rv);
         if (rv.Failed()) {
             return;
         }
 
         element->AppendChild(*sourceElement, rv);
         if (rv.Failed()) {
             return;
         }