--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -13601,8 +13601,24 @@ nsIDocument::ClearStaleServoDataFromDocu
   }
 
   DocumentStyleRootIterator iter(this);
   while (Element* root = iter.GetNextStyleRoot()) {
     ServoRestyleManager::ClearServoDataFromSubtree(root);
   }
   mMightHaveStaleServoData = false;
 }
+
+Selection*
+nsIDocument::GetSelection(ErrorResult& aRv)
+{
+  nsCOMPtr<nsPIDOMWindowInner> window = do_QueryInterface(GetScopeObject());
+  if (!window) {
+    return nullptr;
+  }
+
+  NS_ASSERTION(window->IsInnerWindow(), "Should have inner window here!");
+  if (!window->IsCurrentInnerWindow()) {
+    return nullptr;
+  }
+
+  return nsGlobalWindow::Cast(window)->GetSelection(aRv);
+}

--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -150,16 +150,17 @@ class Location;
 class MediaQueryList;
 class GlobalObject;
 class NodeFilter;
 class NodeIterator;
 enum class OrientationType : uint8_t;
 class ProcessingInstruction;
 class Promise;
 class ScriptLoader;
+class Selection;
 class StyleSheetList;
 class SVGDocument;
 class SVGSVGElement;
 class Touch;
 class TouchList;
 class TreeWalker;
 class XPathEvaluator;
 class XPathExpression;
@@ -938,16 +939,18 @@ public:
    */
   mozilla::dom::DocumentType* GetDoctype() const;
 
   /**
    * Return the root element for this document.
    */
   Element* GetRootElement() const;
 
+  mozilla::dom::Selection* GetSelection(mozilla::ErrorResult& aRv);
+
   /**
    * Gets the event target to dispatch key events to if there is no focused
    * content in the document.
    */
   virtual nsIContent* GetUnfocusedKeyEventTarget();
 
   /**
    * Retrieve information about the viewport as a data structure.

--- a/dom/html/nsHTMLDocument.cpp
+++ b/dom/html/nsHTMLDocument.cpp
@@ -2224,36 +2224,20 @@ nsHTMLDocument::Embeds()
   }
   return mEmbeds;
 }
 
 NS_IMETHODIMP
 nsHTMLDocument::GetSelection(nsISelection** aReturn)
 {
   ErrorResult rv;
-  NS_IF_ADDREF(*aReturn = GetSelection(rv));
+  NS_IF_ADDREF(*aReturn = nsDocument::GetSelection(rv));
   return rv.StealNSResult();
 }
 
-Selection*
-nsHTMLDocument::GetSelection(ErrorResult& aRv)
-{
-  nsCOMPtr<nsPIDOMWindowInner> window = do_QueryInterface(GetScopeObject());
-  if (!window) {
-    return nullptr;
-  }
-
-  NS_ASSERTION(window->IsInnerWindow(), "Should have inner window here!");
-  if (!window->IsCurrentInnerWindow()) {
-    return nullptr;
-  }
-
-  return nsGlobalWindow::Cast(window)->GetSelection(aRv);
-}
-
 NS_IMETHODIMP
 nsHTMLDocument::CaptureEvents()
 {
   WarnOnceAbout(nsIDocument::eUseOfCaptureEvents);
   return NS_OK;
 }
 
 NS_IMETHODIMP

--- a/dom/html/nsHTMLDocument.h
+++ b/dom/html/nsHTMLDocument.h
@@ -246,17 +246,16 @@ public:
   // The XPCOM Get/SetALinkColor work OK for us, since they never throw.
   // The XPCOM Get/SetBgColor work OK for us, since they never throw.
   nsIHTMLCollection* Anchors();
   nsIHTMLCollection* Applets();
   void Clear() const
   {
     // Deprecated
   }
-  mozilla::dom::Selection* GetSelection(mozilla::ErrorResult& aRv);
   // The XPCOM CaptureEvents works fine for us.
   // The XPCOM ReleaseEvents works fine for us.
   // We're picking up GetLocation from Document
   already_AddRefed<mozilla::dom::Location> GetLocation() const
   {
     return nsIDocument::GetLocation();
   }
 

--- a/dom/webidl/Document.webidl
+++ b/dom/webidl/Document.webidl
@@ -425,16 +425,22 @@ partial interface Document {
   /**
    * Removes the element inserted into the CanvasFrame given an AnonymousContent
    * instance.
    */
   [ChromeOnly, Throws]
   void removeAnonymousContent(AnonymousContent aContent);
 };
 
+// http://w3c.github.io/selection-api/#extensions-to-document-interface
+partial interface Document {
+  [Throws]
+  Selection? getSelection();
+};
+
 // Extension to give chrome JS the ability to determine whether
 // the user has interacted with the document or not.
 partial interface Document {
   [ChromeOnly] readonly attribute boolean userHasInteracted;
 };
 
 // Extension to give chrome and XBL JS the ability to determine whether
 // the document is sandboxed without permission to run scripts

--- a/dom/webidl/HTMLDocument.webidl
+++ b/dom/webidl/HTMLDocument.webidl
@@ -69,20 +69,16 @@ interface HTMLDocument : Document {
   readonly attribute HTMLCollection anchors;
   [Pure]
   readonly attribute HTMLCollection applets;
 
   void clear();
 
   readonly attribute HTMLAllCollection all;
 
-  // https://dvcs.w3.org/hg/editing/raw-file/tip/editing.html#selections
-  [Throws]
-  Selection? getSelection();
-
   // @deprecated These are old Netscape 4 methods. Do not use,
   //             the implementation is no-op.
   // XXXbz do we actually need these anymore?
   void                      captureEvents();
   void                      releaseEvents();
 };
 
 partial interface HTMLDocument {