--- a/dom/encoding/TextEncoder.cpp
+++ b/dom/encoding/TextEncoder.cpp
@@ -7,28 +7,21 @@
 #include "nsContentUtils.h"
 #include "nsICharsetConverterManager.h"
 #include "nsServiceManagerUtils.h"
 
 namespace mozilla {
 namespace dom {
 
 void
-TextEncoder::Init(const Optional<nsAString>& aEncoding,
+TextEncoder::Init(const nsAString& aEncoding,
                   ErrorResult& aRv)
 {
-  // If the constructor is called with no arguments, let label be the "utf-8".
-  // Otherwise, let label be the value of the encoding argument.
-  nsAutoString label;
-  if (!aEncoding.WasPassed()) {
-    label.AssignLiteral("utf-8");
-  } else {
-    label.Assign(aEncoding.Value());
-    EncodingUtils::TrimSpaceCharacters(label);
-  }
+  nsAutoString label(aEncoding);
+  EncodingUtils::TrimSpaceCharacters(label);
 
   // Run the steps to get an encoding from Encoding.
   if (!EncodingUtils::FindEncodingForLabel(label, mEncoding)) {
     // If the steps result in failure,
     // throw an "EncodingError" exception and terminate these steps.
     aRv.Throw(NS_ERROR_DOM_ENCODING_NOT_SUPPORTED_ERR);
     return;
   }

--- a/dom/encoding/TextEncoder.h
+++ b/dom/encoding/TextEncoder.h
@@ -23,17 +23,17 @@ class TextEncoder : public nsISupports, 
 {
 public:
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
   NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(TextEncoder)
 
   // The WebIDL constructor.
   static already_AddRefed<TextEncoder>
   Constructor(nsISupports* aGlobal,
-              const Optional<nsAString>& aEncoding,
+              const nsAString& aEncoding,
               ErrorResult& aRv)
   {
     nsRefPtr<TextEncoder> txtEncoder = new TextEncoder(aGlobal);
     txtEncoder->Init(aEncoding, aRv);
     if (aRv.Failed()) {
       return nullptr;
     }
     return txtEncoder.forget();
@@ -93,16 +93,16 @@ private:
    * Validates provided encoding and throws an exception if invalid encoding.
    * If no encoding is provided then mEncoding is default initialised to "utf-8".
    *
    * @param aEncoding    Optional encoding (case insensitive) provided.
    *                     (valid values are "utf-8", "utf-16", "utf-16be")
    *                     Default value is "utf-8" if no encoding is provided.
    * @return aRv         EncodingError exception else null.
    */
-  void Init(const Optional<nsAString>& aEncoding,
+  void Init(const nsAString& aEncoding,
             ErrorResult& aRv);
 };
 
 } // dom
 } // mozilla
 
 #endif // mozilla_dom_textencoder_h_

--- a/dom/webidl/TextDecoder.webidl
+++ b/dom/webidl/TextDecoder.webidl
@@ -1,27 +1,25 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/.
  *
  * The origin of this IDL file is
- * http://wiki.whatwg.org/wiki/StringEncoding
+ * http://encoding.spec.whatwg.org/#interface-textdecoder
  *
- * Copyright © 2006 The WHATWG Contributors
- * http://wiki.whatwg.org/wiki/WHATWG_Wiki:Copyrights
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
  */
 
-[Constructor(optional DOMString encoding = "utf-8",
-             optional TextDecoderOptions options)]
+[Constructor(optional DOMString label = "utf-8", optional TextDecoderOptions options)]
 interface TextDecoder {
   [SetterThrows]
   readonly attribute DOMString encoding;
   [Throws]
-  DOMString decode(optional ArrayBufferView? view = null,
-                   optional TextDecodeOptions options);
+  DOMString decode(optional ArrayBufferView? input = null, optional TextDecodeOptions options);
 };
 
 dictionary TextDecoderOptions {
   boolean fatal = false;
 };
 
 dictionary TextDecodeOptions {
   boolean stream = false;

--- a/dom/webidl/TextEncoder.webidl
+++ b/dom/webidl/TextEncoder.webidl
@@ -1,23 +1,23 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/.
  *
  * The origin of this IDL file is
- * http://wiki.whatwg.org/wiki/StringEncoding
+ * http://encoding.spec.whatwg.org/#interface-textencoder
  *
- * Copyright © 2006 The WHATWG Contributors
- * http://wiki.whatwg.org/wiki/WHATWG_Wiki:Copyrights
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
  */
 
-[Constructor(optional DOMString encoding)]
+[Constructor(optional DOMString label = "utf-8")]
 interface TextEncoder {
   [SetterThrows]
   readonly attribute DOMString encoding;
   [Throws]
-  Uint8Array encode(DOMString? string, optional TextEncodeOptions options);
+  Uint8Array encode(optional DOMString? input = null, optional TextEncodeOptions options);
 };
 
 dictionary TextEncodeOptions {
   boolean stream = false;
 };