--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -8032,18 +8032,18 @@ nsDocument::RetrieveRelevantHeaders(nsIC
 
   mLastModified.Truncate();
   if (modDate != 0) {
     GetFormattedTimeString(modDate, mLastModified);
   }
 }
 
 already_AddRefed<Element>
-nsDocument::CreateElem(const nsAString& aName, nsAtom *aPrefix,
-                       int32_t aNamespaceID, const nsAString* aIs)
+nsIDocument::CreateElem(const nsAString& aName, nsAtom *aPrefix,
+                        int32_t aNamespaceID, const nsAString* aIs)
 {
 #ifdef DEBUG
   nsAutoString qName;
   if (aPrefix) {
     aPrefix->ToString(qName);
     qName.Append(':');
   }
   qName.Append(aName);

--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -270,22 +270,16 @@ public:
   // nsIScriptObjectPrincipal
   virtual nsIPrincipal* GetPrincipal() override;
 
   // nsIApplicationCacheContainer
   NS_DECL_NSIAPPLICATIONCACHECONTAINER
 
   virtual nsresult Init();
 
-  virtual already_AddRefed<Element> CreateElem(const nsAString& aName,
-                                               nsAtom* aPrefix,
-                                               int32_t aNamespaceID,
-                                               const nsAString* aIs = nullptr) override;
-
-
   virtual void Destroy() override;
   virtual void RemovedFromDocShell() override;
 
   virtual void BlockOnload() override;
   virtual void UnblockOnload(bool aFireSync) override;
 
   NS_DECL_CYCLE_COLLECTION_SKIPPABLE_SCRIPT_HOLDER_CLASS_AMBIGUOUS(nsDocument,
                                                                    nsIDocument)

--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -2197,20 +2197,20 @@ public:
   {
     mIsContentDocument = aIsContentDocument;
   }
 
   /**
    * Create an element with the specified name, prefix and namespace ID.
    * Returns null if element name parsing failed.
    */
-  virtual already_AddRefed<Element> CreateElem(const nsAString& aName,
-                                               nsAtom* aPrefix,
-                                               int32_t aNamespaceID,
-                                               const nsAString* aIs = nullptr) = 0;
+  already_AddRefed<Element> CreateElem(const nsAString& aName,
+                                       nsAtom* aPrefix,
+                                       int32_t aNamespaceID,
+                                       const nsAString* aIs = nullptr);
 
   /**
    * Get the security info (i.e. SSL state etc) that the document got
    * from the channel/document that created the content of the
    * document.
    *
    * @see nsIChannel
    */