--- a/dom/webidl/XMLDocument.webidl
+++ b/dom/webidl/XMLDocument.webidl
@@ -11,13 +11,8 @@
 // http://dom.spec.whatwg.org/#xmldocument
 interface XMLDocument : Document {};
 
 // http://www.whatwg.org/specs/web-apps/current-work/#xmldocument
 partial interface XMLDocument {
   [Throws, NeedsCallerType]
   boolean load(DOMString url);
 };
-
-// Gecko extensions?
-partial interface XMLDocument {
-  attribute boolean async;
-};

--- a/dom/xml/XMLDocument.cpp
+++ b/dom/xml/XMLDocument.cpp
@@ -227,17 +227,16 @@ NS_NewXBLDocument(nsIDOMDocument** aInst
 }
 
 namespace mozilla {
 namespace dom {
 
 XMLDocument::XMLDocument(const char* aContentType)
   : nsDocument(aContentType),
     mChannelIsPending(false),
-    mAsync(true),
     mLoopingForSyncLoad(false),
     mIsPlainDocument(false),
     mSuppressParserErrorElement(false),
     mSuppressParserErrorConsoleMessages(false)
 {
   mType = eGenericXML;
 }
 
@@ -456,36 +455,16 @@ XMLDocument::Load(const nsAString& aUrl,
   // Start an asynchronous read of the XML document
   rv = channel->AsyncOpen2(listener);
   if (NS_FAILED(rv)) {
     mChannelIsPending = false;
     aRv.Throw(rv);
     return false;
   }
 
-  if (!mAsync) {
-    nsAutoSyncOperation sync(this);
-    mLoopingForSyncLoad = true;
-    SpinEventLoopUntil([&]() { return !mLoopingForSyncLoad; });
-
-    // We set return to true unless there was a parsing error
-    Element* rootElement = GetRootElement();
-    if (!rootElement) {
-      return false;
-    }
-
-    if (rootElement->LocalName().EqualsLiteral("parsererror")) {
-      nsAutoString ns;
-      rootElement->GetNamespaceURI(ns);
-      if (ns.EqualsLiteral("http://www.mozilla.org/newlayout/xml/parsererror.xml")) {
-        return false;
-      }
-    }
-  }
-
   return true;
 }
 
 void
 XMLDocument::SetSuppressParserErrorElement(bool aSuppress)
 {
   mSuppressParserErrorElement = aSuppress;
 }
@@ -608,17 +587,16 @@ XMLDocument::Clone(mozilla::dom::NodeInf
   NS_ASSERTION(aNodeInfo->NodeInfoManager() == mNodeInfoManager,
                "Can't import this document into another document!");
 
   RefPtr<XMLDocument> clone = new XMLDocument();
   nsresult rv = CloneDocHelper(clone, aPreallocateChildren);
   NS_ENSURE_SUCCESS(rv, rv);
 
   // State from XMLDocument
-  clone->mAsync = mAsync;
   clone->mIsPlainDocument = mIsPlainDocument;
 
   return CallQueryInterface(clone.get(), aResult);
 }
 
 JSObject*
 XMLDocument::WrapNode(JSContext *aCx, JS::Handle<JSObject*> aGivenProto)
 {

--- a/dom/xml/XMLDocument.h
+++ b/dom/xml/XMLDocument.h
@@ -55,24 +55,16 @@ public:
                          bool aPreallocateChildren) const override;
 
   virtual void DocAddSizeOfExcludingThis(nsWindowSizes* aWindowSizes) const override;
   // DocAddSizeOfIncludingThis is inherited from nsIDocument.
 
 
   // WebIDL API
   bool Load(const nsAString& aUrl, CallerType aCallerType, ErrorResult& aRv);
-  bool Async() const
-  {
-    return mAsync;
-  }
-  void SetAsync(bool aAsync)
-  {
-    mAsync = aAsync;
-  }
 
   // .location is [Unforgeable], so we have to make it clear that the
   // nsIDocument version applies to us (it's shadowed by the XPCOM thing on
   // nsDocument).
   using nsIDocument::GetLocation;
 
 protected:
   virtual ~XMLDocument();
@@ -84,17 +76,16 @@ protected:
 
   // mChannelIsPending indicates whether we're currently asynchronously loading
   // data from mChannel (via document.load() or normal load).  It's set to true
   // when we first find out about the channel (StartDocumentLoad) and set to
   // false in EndLoad or if ResetToURI() is called.  In the latter case our
   // mChannel is also cancelled.  Note that if this member is true, mChannel
   // cannot be null.
   bool mChannelIsPending;
-  bool mAsync;
   bool mLoopingForSyncLoad;
 
   // If true. we're really a Document, not an XMLDocument
   bool mIsPlainDocument;
 
   // If true, do not output <parsererror> elements. Per spec, XMLHttpRequest
   // shouldn't output them, whereas DOMParser/others should (see bug 918703).
   bool mSuppressParserErrorElement;

--- a/testing/web-platform/meta/dom/historical.html.ini
+++ b/testing/web-platform/meta/dom/historical.html.ini
@@ -11,11 +11,8 @@
   [Historical DOM features must be removed: createCDATASection]
     expected: FAIL
     bug: 660660
 
   [Node member must be nuked: rootNode]
     disabled: https://bugzilla.mozilla.org/show_bug.cgi?id=1303802
     bug: 1269155
 
-  [Historical DOM features must be removed: async]
-    expected: FAIL
-