--- a/dom/interfaces/css/moz.build
+++ b/dom/interfaces/css/moz.build
@@ -4,17 +4,16 @@
 # 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/.
 
 with Files("**"):
     BUG_COMPONENT = ("Core", "DOM: CSS Object Model")
 
 XPIDL_SOURCES += [
     'nsIDOMCounter.idl',
-    'nsIDOMCSSConditionRule.idl',
     'nsIDOMCSSCounterStyleRule.idl',
     'nsIDOMCSSFontFaceRule.idl',
     'nsIDOMCSSFontFeatureValuesRule.idl',
     'nsIDOMCSSGroupingRule.idl',
     'nsIDOMCSSImportRule.idl',
     'nsIDOMCSSKeyframeRule.idl',
     'nsIDOMCSSKeyframesRule.idl',
     'nsIDOMCSSPageRule.idl',

deleted file mode 100644
--- a/dom/interfaces/css/nsIDOMCSSConditionRule.idl
+++ /dev/null
@@ -1,16 +0,0 @@
-/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
-/* 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/. */
-
-#include "nsIDOMCSSGroupingRule.idl"
-
-/**
- * Interface in the CSS OM for at-rules that conditionally apply their
- * child rules.
- */
-[scriptable, uuid(44da41b2-5660-415d-8692-eae805776103)]
-interface nsIDOMCSSConditionRule : nsIDOMCSSGroupingRule
-{
-  attribute DOMString conditionText;
-};

--- a/layout/style/CSSMediaRule.cpp
+++ b/layout/style/CSSMediaRule.cpp
@@ -13,17 +13,16 @@ namespace mozilla {
 namespace dom {
 
 NS_IMPL_ADDREF_INHERITED(CSSMediaRule, css::ConditionRule)
 NS_IMPL_RELEASE_INHERITED(CSSMediaRule, css::ConditionRule)
 
 // QueryInterface implementation for CSSMediaRule
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(CSSMediaRule)
   NS_INTERFACE_MAP_ENTRY(nsIDOMCSSGroupingRule)
-  NS_INTERFACE_MAP_ENTRY(nsIDOMCSSConditionRule)
 NS_INTERFACE_MAP_END_INHERITING(css::ConditionRule)
 
 // nsIDOMCSSGroupingRule methods
 NS_IMETHODIMP
 CSSMediaRule::GetCssRules(nsIDOMCSSRuleList** aRuleList)
 {
   return GroupRule::GetCssRules(aRuleList);
 }
@@ -36,27 +35,16 @@ CSSMediaRule::InsertRule(const nsAString
 }
 
 NS_IMETHODIMP
 CSSMediaRule::DeleteRule(uint32_t aIndex)
 {
   return GroupRule::DeleteRule(aIndex);
 }
 
-void
-CSSMediaRule::SetConditionText(const nsAString& aConditionText,
-                               ErrorResult& aRv)
-{
-  nsresult rv = static_cast<nsIDOMCSSConditionRule*>(this)->
-    SetConditionText(aConditionText);
-  if (NS_FAILED(rv)) {
-    aRv.Throw(rv);
-  }
-}
-
 /* virtual */ JSObject*
 CSSMediaRule::WrapObject(JSContext* aCx, JS::Handle<JSObject*> aGivenProto)
 {
   return CSSMediaRuleBinding::Wrap(aCx, this, aGivenProto);
 }
 
 } // namespace dom
 } // namespace mozilla

--- a/layout/style/CSSMediaRule.h
+++ b/layout/style/CSSMediaRule.h
@@ -3,47 +3,41 @@
 /* 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/. */
 
 #ifndef mozilla_dom_CSSMediaRule_h
 #define mozilla_dom_CSSMediaRule_h
 
 #include "mozilla/css/GroupRule.h"
-#include "nsIDOMCSSConditionRule.h"
+#include "nsIDOMCSSGroupingRule.h"
 
 namespace mozilla {
 namespace dom {
 
 class CSSMediaRule : public css::ConditionRule
-                   , public nsIDOMCSSConditionRule
+                   , public nsIDOMCSSGroupingRule
 {
 protected:
   using ConditionRule::ConditionRule;
   virtual ~CSSMediaRule() {}
 
 public:
   NS_DECL_ISUPPORTS_INHERITED
 
   int32_t GetType() const override { return css::Rule::MEDIA_RULE; }
 
   // XPCOM interface
   using Rule::GetType;
 
   // nsIDOMCSSGroupingRule interface
   NS_DECL_NSIDOMCSSGROUPINGRULE
 
-  // nsIDOMCSSConditionRule interface
-  NS_IMETHOD SetConditionText(const nsAString& aConditionText) override = 0;
-
   // WebIDL interface
   uint16_t Type() const override { return nsIDOMCSSRule::MEDIA_RULE; }
-  // Our XPCOM GetConditionText is OK
-  void SetConditionText(const nsAString& aConditionText,
-                        ErrorResult& aRv) final;
   virtual MediaList* Media() = 0;
 
   JSObject* WrapObject(JSContext* aCx,
                        JS::Handle<JSObject*> aGivenProto) override;
 };
 
 } // namespace dom
 } // namespace mozilla

--- a/layout/style/CSSMozDocumentRule.cpp
+++ b/layout/style/CSSMozDocumentRule.cpp
@@ -13,17 +13,16 @@ namespace dom {
 using namespace mozilla::css;
 
 NS_IMPL_ADDREF_INHERITED(CSSMozDocumentRule, css::ConditionRule)
 NS_IMPL_RELEASE_INHERITED(CSSMozDocumentRule, css::ConditionRule)
 
 // QueryInterface implementation for CSSMozDocumentRule
 NS_INTERFACE_MAP_BEGIN(CSSMozDocumentRule)
   NS_INTERFACE_MAP_ENTRY(nsIDOMCSSGroupingRule)
-  NS_INTERFACE_MAP_ENTRY(nsIDOMCSSConditionRule)
 NS_INTERFACE_MAP_END_INHERITING(ConditionRule)
 
 // nsIDOMCSSGroupingRule methods
 NS_IMETHODIMP
 CSSMozDocumentRule::GetCssRules(nsIDOMCSSRuleList** aRuleList)
 {
   return GroupRule::GetCssRules(aRuleList);
 }
@@ -36,23 +35,16 @@ CSSMozDocumentRule::InsertRule(const nsA
 }
 
 NS_IMETHODIMP
 CSSMozDocumentRule::DeleteRule(uint32_t aIndex)
 {
   return GroupRule::DeleteRule(aIndex);
 }
 
-void
-CSSMozDocumentRule::SetConditionText(const nsAString& aConditionText,
-                                     ErrorResult& aRv)
-{
-  aRv = SetConditionText(aConditionText);
-}
-
 /* virtual */ JSObject*
 CSSMozDocumentRule::WrapObject(JSContext* aCx,
                                JS::Handle<JSObject*> aGivenProto)
 {
   return CSSMozDocumentRuleBinding::Wrap(aCx, this, aGivenProto);
 }
 
 bool

--- a/layout/style/CSSMozDocumentRule.h
+++ b/layout/style/CSSMozDocumentRule.h
@@ -9,17 +9,17 @@
 
 #include "mozilla/css/GroupRule.h"
 #include "mozilla/css/URLMatchingFunction.h"
 
 namespace mozilla {
 namespace dom {
 
 class CSSMozDocumentRule : public css::ConditionRule
-                         , public nsIDOMCSSConditionRule
+                         , public nsIDOMCSSGroupingRule
 {
 protected:
   using ConditionRule::ConditionRule;
   virtual ~CSSMozDocumentRule() {}
 
 public:
   NS_DECL_ISUPPORTS_INHERITED
 
@@ -30,26 +30,20 @@ public:
                     nsIURI* aDocURI,
                     const nsACString& aDocURISpec,
                     const nsACString& aPattern,
                     css::URLMatchingFunction aUrlMatchingFunction);
 
   // nsIDOMCSSGroupingRule interface
   NS_DECL_NSIDOMCSSGROUPINGRULE
 
-  // nsIDOMCSSConditionRule interface
-  NS_IMETHOD SetConditionText(const nsAString& aConditionText) override = 0;
-
   // WebIDL interface
   uint16_t Type() const final override {
     return nsIDOMCSSRule::DOCUMENT_RULE;
   }
-  // Our XPCOM GetConditionText is OK
-  void SetConditionText(const nsAString& aConditionText,
-                        ErrorResult& aRv) final;
 
   JSObject* WrapObject(JSContext* aCx,
                        JS::Handle<JSObject*> aGivenProto) override;
 };
 
 } // namespace dom
 } // namespace mozilla
 

--- a/layout/style/CSSSupportsRule.cpp
+++ b/layout/style/CSSSupportsRule.cpp
@@ -14,17 +14,16 @@ namespace mozilla {
 namespace dom {
 
 NS_IMPL_ADDREF_INHERITED(mozilla::dom::CSSSupportsRule, css::ConditionRule)
 NS_IMPL_RELEASE_INHERITED(mozilla::dom::CSSSupportsRule, css::ConditionRule)
 
 // QueryInterface implementation for CSSSupportsRule
 NS_INTERFACE_MAP_BEGIN(CSSSupportsRule)
   NS_INTERFACE_MAP_ENTRY(nsIDOMCSSGroupingRule)
-  NS_INTERFACE_MAP_ENTRY(nsIDOMCSSConditionRule)
 NS_INTERFACE_MAP_END_INHERITING(ConditionRule)
 
 // nsIDOMCSSGroupingRule methods
 NS_IMETHODIMP
 CSSSupportsRule::GetCssRules(nsIDOMCSSRuleList** aRuleList)
 {
   return GroupRule::GetCssRules(aRuleList);
 }
@@ -36,23 +35,16 @@ CSSSupportsRule::InsertRule(const nsAStr
 }
 
 NS_IMETHODIMP
 CSSSupportsRule::DeleteRule(uint32_t aIndex)
 {
   return GroupRule::DeleteRule(aIndex);
 }
 
-void
-CSSSupportsRule::SetConditionText(const nsAString& aConditionText,
-                                  ErrorResult& aRv)
-{
-  aRv = SetConditionText(aConditionText);
-}
-
 /* virtual */ JSObject*
 CSSSupportsRule::WrapObject(JSContext* aCx,
                             JS::Handle<JSObject*> aGivenProto)
 {
   return CSSSupportsRuleBinding::Wrap(aCx, this, aGivenProto);
 }
 
 } // namespace dom

--- a/layout/style/CSSSupportsRule.h
+++ b/layout/style/CSSSupportsRule.h
@@ -3,45 +3,39 @@
 /* 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/. */
 
 #ifndef mozilla_dom_CSSSupportsRule_h
 #define mozilla_dom_CSSSupportsRule_h
 
 #include "mozilla/css/GroupRule.h"
-#include "nsIDOMCSSConditionRule.h"
+#include "nsIDOMCSSGroupingRule.h"
 
 namespace mozilla {
 namespace dom {
 
 class CSSSupportsRule : public css::ConditionRule
-                      , public nsIDOMCSSConditionRule
+                      , public nsIDOMCSSGroupingRule
 {
 protected:
   using ConditionRule::ConditionRule;
   virtual ~CSSSupportsRule() {}
 
 public:
   NS_DECL_ISUPPORTS_INHERITED
 
   int32_t GetType() const override { return css::Rule::SUPPORTS_RULE; }
   using Rule::GetType;
 
   // nsIDOMCSSGroupingRule interface
   NS_DECL_NSIDOMCSSGROUPINGRULE
 
-  // nsIDOMCSSConditionRule interface
-  NS_IMETHOD SetConditionText(const nsAString& aConditionText) override = 0;
-
   // WebIDL interface
   uint16_t Type() const override { return nsIDOMCSSRule::SUPPORTS_RULE; }
-  // Our XPCOM GetConditionText is OK
-  void SetConditionText(const nsAString& aConditionText,
-                        ErrorResult& aRv) final;
 
   JSObject* WrapObject(JSContext* aCx,
                        JS::Handle<JSObject*> aGivenProto) override;
 };
 
 } // namespace dom
 } // namespace mozilla
 

--- a/layout/style/GroupRule.h
+++ b/layout/style/GroupRule.h
@@ -226,20 +226,17 @@ private:
 
 // Implementation of WebIDL CSSConditionRule.
 class ConditionRule : public GroupRule
 {
 protected:
   using GroupRule::GroupRule;
 
 public:
-  // GetConditionText signature matches nsIDOMCSSConditionRule, so subclasses
-  // can implement this easily.  The implementations should never return
-  // anything other than NS_OK.
-  NS_IMETHOD GetConditionText(nsAString& aConditionText) = 0;
+  virtual void GetConditionText(nsAString& aConditionText) = 0;
   virtual void SetConditionText(const nsAString& aConditionText,
                                 ErrorResult& aRv) = 0;
 };
 
 } // namespace css
 } // namespace mozilla
 
 #endif /* mozilla_css_GroupRule_h__ */

--- a/layout/style/ServoDocumentRule.cpp
+++ b/layout/style/ServoDocumentRule.cpp
@@ -60,29 +60,27 @@ ServoDocumentRule::List(FILE* out, int32
   for (int32_t i = 0; i < aIndent; i++) {
     str.AppendLiteral("  ");
   }
   Servo_DocumentRule_Debug(mRawRule, &str);
   fprintf_stderr(out, "%s\n", str.get());
 }
 #endif
 
-// nsIDOMCSSConditionRule methods
-
-NS_IMETHODIMP
+void
 ServoDocumentRule::GetConditionText(nsAString& aConditionText)
 {
   Servo_DocumentRule_GetConditionText(mRawRule, &aConditionText);
-  return NS_OK;
 }
 
-NS_IMETHODIMP
-ServoDocumentRule::SetConditionText(const nsAString& aConditionText)
+void
+ServoDocumentRule::SetConditionText(const nsAString& aConditionText,
+                                    ErrorResult& aRv)
 {
-  return NS_ERROR_NOT_IMPLEMENTED;
+  aRv.Throw(NS_ERROR_NOT_IMPLEMENTED);
 }
 
 /* virtual */ void
 ServoDocumentRule::GetCssTextImpl(nsAString& aCssText) const
 {
   Servo_DocumentRule_GetCssText(mRawRule, &aCssText);
 }
 

--- a/layout/style/ServoDocumentRule.h
+++ b/layout/style/ServoDocumentRule.h
@@ -26,22 +26,21 @@ public:
   bool UseForPresentation(nsPresContext* aPresContext,
                           nsMediaQueryResultCacheKey& aKey) final;
 #ifdef DEBUG
   void List(FILE* out = stdout, int32_t aIndent = 0) const final;
 #endif
 
   RawServoDocumentRule* Raw() const { return mRawRule; }
 
-  // nsIDOMCSSConditionRule interface
-  NS_DECL_NSIDOMCSSCONDITIONRULE
-
   // WebIDL interface
-  void GetCssTextImpl(nsAString& aCssText) const override;
-  using CSSMozDocumentRule::SetConditionText;
+  void GetCssTextImpl(nsAString& aCssText) const final;
+  void GetConditionText(nsAString& aConditionText) final;
+  void SetConditionText(const nsAString& aConditionText,
+                        ErrorResult& aRv) final;
 
   size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf)
     const override;
 
 private:
   virtual ~ServoDocumentRule();
 
   RefPtr<RawServoDocumentRule> mRawRule;

--- a/layout/style/ServoMediaRule.cpp
+++ b/layout/style/ServoMediaRule.cpp
@@ -86,28 +86,27 @@ ServoMediaRule::List(FILE* out, int32_t 
   for (int32_t i = 0; i < aIndent; i++) {
     str.AppendLiteral("  ");
   }
   Servo_MediaRule_Debug(mRawRule, &str);
   fprintf_stderr(out, "%s\n", str.get());
 }
 #endif
 
-// nsIDOMCSSConditionRule methods
-
-NS_IMETHODIMP
+void
 ServoMediaRule::GetConditionText(nsAString& aConditionText)
 {
-  return Media()->GetMediaText(aConditionText);
+  Media()->GetMediaText(aConditionText);
 }
 
-NS_IMETHODIMP
-ServoMediaRule::SetConditionText(const nsAString& aConditionText)
+void
+ServoMediaRule::SetConditionText(const nsAString& aConditionText,
+                                 ErrorResult& aRv)
 {
-  return Media()->SetMediaText(aConditionText);
+  aRv = Media()->SetMediaText(aConditionText);
 }
 
 /* virtual */ void
 ServoMediaRule::GetCssTextImpl(nsAString& aCssText) const
 {
   Servo_MediaRule_GetCssText(mRawRule, &aCssText);
 }
 

--- a/layout/style/ServoMediaRule.h
+++ b/layout/style/ServoMediaRule.h
@@ -30,23 +30,22 @@ public:
                           nsMediaQueryResultCacheKey& aKey) final;
   void SetStyleSheet(StyleSheet* aSheet) override;
 #ifdef DEBUG
   void List(FILE* out = stdout, int32_t aIndent = 0) const final;
 #endif
 
   RawServoMediaRule* Raw() const { return mRawRule; }
 
-  // nsIDOMCSSConditionRule interface
-  NS_DECL_NSIDOMCSSCONDITIONRULE
-
   // WebIDL interface
-  void GetCssTextImpl(nsAString& aCssText) const override;
-  using CSSMediaRule::SetConditionText;
-  dom::MediaList* Media() override;
+  void GetCssTextImpl(nsAString& aCssText) const final;
+  void GetConditionText(nsAString& aConditionText) final;
+  void SetConditionText(const nsAString& aConditionText,
+                        ErrorResult& aRv) final;
+  dom::MediaList* Media() final;
 
   size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf)
     const override;
 
 private:
   virtual ~ServoMediaRule();
 
   RefPtr<RawServoMediaRule> mRawRule;

--- a/layout/style/ServoSupportsRule.cpp
+++ b/layout/style/ServoSupportsRule.cpp
@@ -61,29 +61,27 @@ ServoSupportsRule::List(FILE* out, int32
   for (int32_t i = 0; i < aIndent; i++) {
     str.AppendLiteral("  ");
   }
   Servo_SupportsRule_Debug(mRawRule, &str);
   fprintf_stderr(out, "%s\n", str.get());
 }
 #endif
 
-// nsIDOMCSSConditionRule methods
-
-NS_IMETHODIMP
+void
 ServoSupportsRule::GetConditionText(nsAString& aConditionText)
 {
   Servo_SupportsRule_GetConditionText(mRawRule, &aConditionText);
-  return NS_OK;
 }
 
-NS_IMETHODIMP
-ServoSupportsRule::SetConditionText(const nsAString& aConditionText)
+void
+ServoSupportsRule::SetConditionText(const nsAString& aConditionText,
+                                    ErrorResult& aRv)
 {
-  return NS_ERROR_NOT_IMPLEMENTED;
+  aRv.Throw(NS_ERROR_NOT_IMPLEMENTED);
 }
 
 /* virtual */ void
 ServoSupportsRule::GetCssTextImpl(nsAString& aCssText) const
 {
   Servo_SupportsRule_GetCssText(mRawRule, &aCssText);
 }
 

--- a/layout/style/ServoSupportsRule.h
+++ b/layout/style/ServoSupportsRule.h
@@ -26,22 +26,21 @@ public:
   bool UseForPresentation(nsPresContext* aPresContext,
                           nsMediaQueryResultCacheKey& aKey) final;
 #ifdef DEBUG
   void List(FILE* out = stdout, int32_t aIndent = 0) const final;
 #endif
 
   RawServoSupportsRule* Raw() const { return mRawRule; }
 
-  // nsIDOMCSSConditionRule interface
-  NS_DECL_NSIDOMCSSCONDITIONRULE
-
   // WebIDL interface
-  void GetCssTextImpl(nsAString& aCssText) const override;
-  using CSSSupportsRule::SetConditionText;
+  void GetCssTextImpl(nsAString& aCssText) const final;
+  void GetConditionText(nsAString& aConditionText) final;
+  void SetConditionText(const nsAString& aConditionText,
+                        ErrorResult& aRv) final;
 
   size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf)
     const override;
 
 private:
   virtual ~ServoSupportsRule();
 
   RefPtr<RawServoSupportsRule> mRawRule;

--- a/layout/style/nsCSSRules.cpp
+++ b/layout/style/nsCSSRules.cpp
@@ -318,39 +318,40 @@ MediaRule::Media()
 void
 MediaRule::GetCssTextImpl(nsAString& aCssText) const
 {
   aCssText.AssignLiteral("@media ");
   AppendConditionText(aCssText);
   GroupRule::AppendRulesToCssText(aCssText);
 }
 
-// nsIDOMCSSConditionRule methods
-NS_IMETHODIMP
+void
 MediaRule::GetConditionText(nsAString& aConditionText)
 {
   aConditionText.Truncate(0);
   AppendConditionText(aConditionText);
-  return NS_OK;
 }
 
-NS_IMETHODIMP
-MediaRule::SetConditionText(const nsAString& aConditionText)
+void
+MediaRule::SetConditionText(const nsAString& aConditionText,
+                            ErrorResult& aRv)
 {
   if (!mMedia) {
     RefPtr<nsMediaList> media = new nsMediaList();
     media->SetStyleSheet(GetStyleSheet());
     nsresult rv = media->SetMediaText(aConditionText);
     if (NS_SUCCEEDED(rv)) {
       mMedia = media;
+    } else {
+      aRv.Throw(rv);
     }
-    return rv;
+    return;
   }
 
-  return mMedia->SetMediaText(aConditionText);
+  aRv = mMedia->SetMediaText(aConditionText);
 }
 
 // GroupRule interface
 /* virtual */ bool
 MediaRule::UseForPresentation(nsPresContext* aPresContext,
                               nsMediaQueryResultCacheKey& aKey)
 {
   if (mMedia) {
@@ -455,29 +456,28 @@ DocumentRule::Clone() const
 void
 DocumentRule::GetCssTextImpl(nsAString& aCssText) const
 {
   aCssText.AssignLiteral("@-moz-document ");
   AppendConditionText(aCssText);
   GroupRule::AppendRulesToCssText(aCssText);
 }
 
-// nsIDOMCSSConditionRule methods
-NS_IMETHODIMP
+void
 DocumentRule::GetConditionText(nsAString& aConditionText)
 {
   aConditionText.Truncate(0);
   AppendConditionText(aConditionText);
-  return NS_OK;
 }
 
-NS_IMETHODIMP
-DocumentRule::SetConditionText(const nsAString& aConditionText)
+void
+DocumentRule::SetConditionText(const nsAString& aConditionText,
+                               ErrorResult& aRv)
 {
-  return NS_ERROR_NOT_IMPLEMENTED;
+  aRv.Throw(NS_ERROR_NOT_IMPLEMENTED);
 }
 
 // GroupRule interface
 /* virtual */ bool
 DocumentRule::UseForPresentation(nsPresContext* aPresContext,
                                  nsMediaQueryResultCacheKey& aKey)
 {
   return UseForPresentation(aPresContext);
@@ -1541,28 +1541,27 @@ NS_INTERFACE_MAP_END_INHERITING(dom::CSS
 void
 CSSSupportsRule::GetCssTextImpl(nsAString& aCssText) const
 {
   aCssText.AssignLiteral("@supports ");
   aCssText.Append(mCondition);
   css::GroupRule::AppendRulesToCssText(aCssText);
 }
 
-// nsIDOMCSSConditionRule methods
-NS_IMETHODIMP
+void
 CSSSupportsRule::GetConditionText(nsAString& aConditionText)
 {
   aConditionText.Assign(mCondition);
-  return NS_OK;
 }
 
-NS_IMETHODIMP
-CSSSupportsRule::SetConditionText(const nsAString& aConditionText)
+void
+CSSSupportsRule::SetConditionText(const nsAString& aConditionText,
+                                  ErrorResult& aRv)
 {
-  return NS_ERROR_NOT_IMPLEMENTED;
+  aRv.Throw(NS_ERROR_NOT_IMPLEMENTED);
 }
 
 /* virtual */ size_t
 CSSSupportsRule::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
 {
   size_t n = aMallocSizeOf(this);
   n += css::GroupRule::SizeOfExcludingThis(aMallocSizeOf);
   n += mCondition.SizeOfExcludingThisIfUnshared(aMallocSizeOf);

--- a/layout/style/nsCSSRules.h
+++ b/layout/style/nsCSSRules.h
@@ -24,17 +24,16 @@
 #include "mozilla/dom/CSSMediaRule.h"
 #include "mozilla/dom/CSSPageRule.h"
 #include "mozilla/dom/CSSSupportsRule.h"
 #include "mozilla/dom/CSSMozDocumentRule.h"
 #include "nsAutoPtr.h"
 #include "nsCSSPropertyID.h"
 #include "nsCSSValue.h"
 #include "nsDOMCSSDeclaration.h"
-#include "nsIDOMCSSConditionRule.h"
 #include "nsIDOMCSSFontFeatureValuesRule.h"
 #include "nsIDOMCSSGroupingRule.h"
 #include "nsTArray.h"
 
 class nsMediaList;
 
 namespace mozilla {
 
@@ -65,30 +64,29 @@ public:
   virtual void SetStyleSheet(mozilla::StyleSheet* aSheet) override; //override GroupRule
   mozilla::CSSStyleSheet* GetStyleSheet() const
   {
     mozilla::StyleSheet* sheet = GroupRule::GetStyleSheet();
     return sheet ? sheet->AsGecko() : nullptr;
   }
   virtual already_AddRefed<Rule> Clone() const override;
 
-  // nsIDOMCSSConditionRule interface
-  NS_DECL_NSIDOMCSSCONDITIONRULE
-
   // rest of GroupRule
   virtual bool UseForPresentation(nsPresContext* aPresContext,
                                     nsMediaQueryResultCacheKey& aKey) override;
 
   // @media rule methods
   nsresult SetMedia(nsMediaList* aMedia);
 
   // WebIDL interface
-  void GetCssTextImpl(nsAString& aCssText) const override;
-  using CSSMediaRule::SetConditionText;
-  dom::MediaList* Media() override;
+  void GetCssTextImpl(nsAString& aCssText) const final;
+  void GetConditionText(nsAString& aConditionText) final;
+  void SetConditionText(const nsAString& aConditionText,
+                        ErrorResult& aRv) final;
+  dom::MediaList* Media() final;
 
   virtual size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf)
     const override MOZ_MUST_OVERRIDE;
 
 protected:
   void AppendConditionText(nsAString& aOutput) const;
 
   RefPtr<nsMediaList> mMedia;
@@ -106,19 +104,16 @@ public:
   NS_DECL_ISUPPORTS_INHERITED
 
   // Rule methods
 #ifdef DEBUG
   virtual void List(FILE* out = stdout, int32_t aIndent = 0) const override;
 #endif
   virtual already_AddRefed<Rule> Clone() const override;
 
-  // nsIDOMCSSConditionRule interface
-  NS_DECL_NSIDOMCSSCONDITIONRULE
-
   // rest of GroupRule
   virtual bool UseForPresentation(nsPresContext* aPresContext,
                                   nsMediaQueryResultCacheKey& aKey) override;
 
   bool UseForPresentation(nsPresContext* aPresContext);
 
   struct URL {
     URLMatchingFunction func;
@@ -133,18 +128,20 @@ public:
     {
     }
     ~URL();
   };
 
   void SetURLs(URL *aURLs) { mURLs = aURLs; }
 
   // WebIDL interface
-  void GetCssTextImpl(nsAString& aCssText) const override;
-  using dom::CSSMozDocumentRule::SetConditionText;
+  void GetCssTextImpl(nsAString& aCssText) const final;
+  void GetConditionText(nsAString& aConditionText) final;
+  void SetConditionText(const nsAString& aConditionText,
+                        ErrorResult& aRv) final;
 
   virtual size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf)
     const override MOZ_MUST_OVERRIDE;
 
 protected:
   void AppendConditionText(nsAString& aOutput) const;
 
   nsAutoPtr<URL> mURLs; // linked list of |struct URL| above.
@@ -417,22 +414,21 @@ public:
   virtual void List(FILE* out = stdout, int32_t aIndent = 0) const override;
 #endif
   virtual already_AddRefed<mozilla::css::Rule> Clone() const override;
   virtual bool UseForPresentation(nsPresContext* aPresContext,
                                   nsMediaQueryResultCacheKey& aKey) override;
 
   NS_DECL_ISUPPORTS_INHERITED
 
-  // nsIDOMCSSConditionRule interface
-  NS_DECL_NSIDOMCSSCONDITIONRULE
-
   // WebIDL interface
-  void GetCssTextImpl(nsAString& aCssText) const override;
-  using dom::CSSSupportsRule::SetConditionText;
+  void GetCssTextImpl(nsAString& aCssText) const final;
+  void GetConditionText(nsAString& aConditionText) final;
+  void SetConditionText(const nsAString& aConditionText,
+                        ErrorResult& aRv) final;
 
   virtual size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const override;
 
 protected:
   virtual ~CSSSupportsRule();
 
   bool mUseGroup;
   nsString mCondition;