--- a/dom/interfaces/css/moz.build
+++ b/dom/interfaces/css/moz.build
@@ -20,17 +20,16 @@ XPIDL_SOURCES += [
     'nsIDOMCSSMediaRule.idl',
     'nsIDOMCSSPageRule.idl',
     'nsIDOMCSSPrimitiveValue.idl',
     'nsIDOMCSSRule.idl',
     'nsIDOMCSSRuleList.idl',
     'nsIDOMCSSStyleDeclaration.idl',
     'nsIDOMCSSStyleRule.idl',
     'nsIDOMCSSStyleSheet.idl',
-    'nsIDOMCSSSupportsRule.idl',
     'nsIDOMCSSUnknownRule.idl',
     'nsIDOMCSSValue.idl',
     'nsIDOMCSSValueList.idl',
     'nsIDOMRect.idl',
 ]
 
 XPIDL_MODULE = 'dom_css'
 

deleted file mode 100644
--- a/dom/interfaces/css/nsIDOMCSSSupportsRule.idl
+++ /dev/null
@@ -1,14 +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 "nsIDOMCSSConditionRule.idl"
-
-/**
- * Interface for @supports rules in the CSS OM.
- */
-[scriptable, uuid(0b9e63a1-1bd7-4caf-850e-148b762b14d2)]
-interface nsIDOMCSSSupportsRule : nsIDOMCSSConditionRule
-{
-};

--- a/layout/style/CSSSupportsRule.cpp
+++ b/layout/style/CSSSupportsRule.cpp
@@ -15,17 +15,16 @@ 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_ENTRY(nsIDOMCSSSupportsRule)
 NS_INTERFACE_MAP_END_INHERITING(ConditionRule)
 
 // nsIDOMCSSGroupingRule methods
 NS_IMETHODIMP
 CSSSupportsRule::GetCssRules(nsIDOMCSSRuleList** aRuleList)
 {
   return GroupRule::GetCssRules(aRuleList);
 }

--- a/layout/style/CSSSupportsRule.h
+++ b/layout/style/CSSSupportsRule.h
@@ -3,23 +3,23 @@
 /* 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/. */
 
 #ifndef mozilla_dom_CSSSupportsRule_h
 #define mozilla_dom_CSSSupportsRule_h
 
 #include "mozilla/css/GroupRule.h"
-#include "nsIDOMCSSSupportsRule.h"
+#include "nsIDOMCSSConditionRule.h"
 
 namespace mozilla {
 namespace dom {
 
 class CSSSupportsRule : public css::ConditionRule
-                      , public nsIDOMCSSSupportsRule
+                      , public nsIDOMCSSConditionRule
 {
 protected:
   using ConditionRule::ConditionRule;
   virtual ~CSSSupportsRule() {}
 
 public:
   NS_DECL_ISUPPORTS_INHERITED
 
@@ -27,19 +27,16 @@ public:
   using Rule::GetType;
 
   // nsIDOMCSSGroupingRule interface
   NS_DECL_NSIDOMCSSGROUPINGRULE
 
   // nsIDOMCSSConditionRule interface
   NS_IMETHOD SetConditionText(const nsAString& aConditionText) override = 0;
 
-  // nsIDOMCSSSupportsRule interface
-  NS_DECL_NSIDOMCSSSUPPORTSRULE
-
   // 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;

--- a/layout/style/nsCSSRules.h
+++ b/layout/style/nsCSSRules.h
@@ -27,17 +27,16 @@
 #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 "nsIDOMCSSSupportsRule.h"
 #include "nsTArray.h"
 
 class nsMediaList;
 
 namespace mozilla {
 
 class ErrorResult;