--- a/dom/bindings/Bindings.conf
+++ b/dom/bindings/Bindings.conf
@@ -208,20 +208,16 @@ DOMInterfaces = {
     'headerFile': 'nsCSSRules.h',
 },
 
 'CSSGroupingRule': {
     'concrete': False,
     'nativeType': 'mozilla::css::GroupRule',
 },
 
-'CSSImportRule': {
-    'nativeType': 'mozilla::css::ImportRule',
-},
-
 'CSSLexer': {
     'wrapperCache': False
 },
 
 'CSSPrimitiveValue': {
     'nativeType': 'nsROCSSPrimitiveValue',
 },
 

new file mode 100644
--- /dev/null
+++ b/layout/style/CSSImportRule.cpp
@@ -0,0 +1,54 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "mozilla/dom/CSSImportRule.h"
+
+#include "mozilla/dom/CSSImportRuleBinding.h"
+#include "mozilla/dom/MediaList.h"
+
+namespace mozilla {
+namespace dom {
+
+NS_IMPL_ADDREF_INHERITED(CSSImportRule, css::Rule)
+NS_IMPL_RELEASE_INHERITED(CSSImportRule, css::Rule)
+
+// QueryInterface implementation for CSSImportRule
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION_INHERITED(CSSImportRule)
+  NS_INTERFACE_MAP_ENTRY(nsIDOMCSSImportRule)
+NS_INTERFACE_MAP_END_INHERITING(css::Rule)
+
+bool
+CSSImportRule::IsCCLeaf() const
+{
+  // We're not a leaf.
+  return false;
+}
+
+NS_IMETHODIMP
+CSSImportRule::GetMedia(nsIDOMMediaList** aMedia)
+{
+  NS_ENSURE_ARG_POINTER(aMedia);
+  NS_ADDREF(*aMedia = Media());
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+CSSImportRule::GetStyleSheet(nsIDOMCSSStyleSheet** aStyleSheet)
+{
+  NS_ENSURE_ARG_POINTER(aStyleSheet);
+  NS_IF_ADDREF(*aStyleSheet = GetStyleSheet());
+  return NS_OK;
+}
+
+/* virtual */ JSObject*
+CSSImportRule::WrapObject(JSContext* aCx,
+                          JS::Handle<JSObject*> aGivenProto)
+{
+  return CSSImportRuleBinding::Wrap(aCx, this, aGivenProto);
+}
+
+} // namespace dom
+} // namespace mozilla

new file mode 100644
--- /dev/null
+++ b/layout/style/CSSImportRule.h
@@ -0,0 +1,50 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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_CSSImportRule_h
+#define mozilla_dom_CSSImportRule_h
+
+#include "mozilla/css/Rule.h"
+#include "nsIDOMCSSImportRule.h"
+
+namespace mozilla {
+namespace dom {
+
+class CSSImportRule : public css::Rule
+                    , public nsIDOMCSSImportRule
+{
+protected:
+  using Rule::Rule;
+  virtual ~CSSImportRule() {}
+
+public:
+  NS_DECL_ISUPPORTS_INHERITED
+  bool IsCCLeaf() const final;
+
+  int32_t GetType() const final { return css::Rule::IMPORT_RULE; }
+  using Rule::GetType;
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf)
+    const override = 0;
+
+  // nsIDOMCSSImportRule interface
+  NS_IMETHOD GetMedia(nsIDOMMediaList** aMedia) final;
+  NS_IMETHOD GetStyleSheet(nsIDOMCSSStyleSheet** aStyleSheet) final;
+
+  // WebIDL interface
+  uint16_t Type() const final { return nsIDOMCSSRule::IMPORT_RULE; }
+  // The XPCOM GetHref is fine, since it never fails.
+  virtual dom::MediaList* Media() const = 0;
+  virtual StyleSheet* GetStyleSheet() const = 0;
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+};
+
+} // namespace dom
+} // namespace mozilla
+
+#endif // mozilla_dom_CSSImportRule_h

--- a/layout/style/ImportRule.h
+++ b/layout/style/ImportRule.h
@@ -6,74 +6,65 @@
 /* class for CSS @import rules */
 
 #ifndef mozilla_css_ImportRule_h__
 #define mozilla_css_ImportRule_h__
 
 #include "mozilla/Attributes.h"
 
 #include "mozilla/MemoryReporting.h"
-#include "mozilla/css/Rule.h"
-#include "nsIDOMCSSImportRule.h"
+#include "mozilla/dom/CSSImportRule.h"
 
 class nsMediaList;
 class nsString;
 
 namespace mozilla {
 
 class CSSStyleSheet;
 class StyleSheet;
 
 namespace dom {
 class MediaList;
 }
 
 namespace css {
 
-class ImportRule final : public Rule,
-                         public nsIDOMCSSImportRule
+class ImportRule final : public dom::CSSImportRule
 {
 public:
   ImportRule(nsMediaList* aMedia, const nsString& aURLSpec,
              uint32_t aLineNumber, uint32_t aColumnNumber);
 private:
   // for |Clone|
   ImportRule(const ImportRule& aCopy);
   ~ImportRule();
 public:
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ImportRule, Rule)
   NS_DECL_ISUPPORTS_INHERITED
-  virtual bool IsCCLeaf() const override;
 
-  using Rule::GetStyleSheet; // unhide since nsIDOMCSSImportRule has its own GetStyleSheet
+  // unhide since nsIDOMCSSImportRule has its own GetStyleSheet
+  using dom::CSSImportRule::GetStyleSheet;
 
   // Rule methods
 #ifdef DEBUG
   virtual void List(FILE* out = stdout, int32_t aIndent = 0) const override;
 #endif
-  virtual int32_t GetType() const override;
-  using Rule::GetType;
   virtual already_AddRefed<Rule> Clone() const override;
 
   void SetSheet(CSSStyleSheet*);
 
   virtual size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const override;
 
-  virtual JSObject* WrapObject(JSContext* aCx,
-                               JS::Handle<JSObject*> aGivenProto) override;
-
   // nsIDOMCSSImportRule interface
-  NS_DECL_NSIDOMCSSIMPORTRULE
+  NS_IMETHOD GetHref(nsAString& aHref) final;
 
   // WebIDL interface
-  uint16_t Type() const override;
   void GetCssTextImpl(nsAString& aCssText) const override;
-  // The XPCOM GetHref is fine, since it never fails.
-  dom::MediaList* Media() const;
-  StyleSheet* GetStyleSheet() const;
+  dom::MediaList* Media() const final;
+  StyleSheet* GetStyleSheet() const final;
 
 private:
   nsString  mURLSpec;
   RefPtr<nsMediaList> mMedia;
   RefPtr<CSSStyleSheet> mChildSheet;
 };
 
 } // namespace css

--- a/layout/style/moz.build
+++ b/layout/style/moz.build
@@ -131,16 +131,17 @@ EXPORTS.mozilla += [
     'StyleSheet.h',
     'StyleSheetInfo.h',
     'StyleSheetInlines.h',
     'URLExtraData.h',
 ]
 
 EXPORTS.mozilla.dom += [
     'CSS.h',
+    'CSSImportRule.h',
     'CSSKeyframeRule.h',
     'CSSKeyframesRule.h',
     'CSSLexer.h',
     'CSSMediaRule.h',
     'CSSMozDocumentRule.h',
     'CSSNamespaceRule.h',
     'CSSPageRule.h',
     'CSSRuleList.h',
@@ -167,16 +168,17 @@ EXPORTS.mozilla.css += [
     'URLMatchingFunction.h',
 ]
 
 UNIFIED_SOURCES += [
     'AnimationCollection.cpp',
     'BindingStyleRule.cpp',
     'CounterStyleManager.cpp',
     'CSS.cpp',
+    'CSSImportRule.cpp',
     'CSSKeyframeRule.cpp',
     'CSSKeyframesRule.cpp',
     'CSSLexer.cpp',
     'CSSMediaRule.cpp',
     'CSSMozDocumentRule.cpp',
     'CSSPageRule.cpp',
     'CSSRuleList.cpp',
     'CSSStyleSheet.cpp',

--- a/layout/style/nsCSSRules.cpp
+++ b/layout/style/nsCSSRules.cpp
@@ -31,17 +31,16 @@
 
 #include "nsContentUtils.h"
 #include "nsError.h"
 #include "nsStyleUtil.h"
 #include "mozilla/DeclarationBlockInlines.h"
 #include "nsCSSParser.h"
 #include "nsDOMClassInfoID.h"
 #include "mozilla/dom/CSSStyleDeclarationBinding.h"
-#include "mozilla/dom/CSSImportRuleBinding.h"
 #include "mozilla/dom/CSSFontFaceRuleBinding.h"
 #include "mozilla/dom/CSSFontFeatureValuesRuleBinding.h"
 #include "mozilla/dom/CSSCounterStyleRuleBinding.h"
 #include "StyleRule.h"
 #include "nsFont.h"
 #include "nsIURI.h"
 #include "mozAutoDocUpdate.h"
 #include "nsCCUncollectableMarker.h"
@@ -168,28 +167,28 @@ Rule::GetParentRule() const
 }
 
 // -------------------------------------------
 // ImportRule
 //
 
 ImportRule::ImportRule(nsMediaList* aMedia, const nsString& aURLSpec,
                        uint32_t aLineNumber, uint32_t aColumnNumber)
-  : Rule(aLineNumber, aColumnNumber)
+  : CSSImportRule(aLineNumber, aColumnNumber)
   , mURLSpec(aURLSpec)
   , mMedia(aMedia)
 {
   MOZ_ASSERT(aMedia);
   // XXXbz This is really silly.... the mMedia here will be replaced
   // with itself if we manage to load a sheet.  Which should really
   // never fail nowadays, in sane cases.
 }
 
 ImportRule::ImportRule(const ImportRule& aCopy)
-  : Rule(aCopy),
+  : CSSImportRule(aCopy),
     mURLSpec(aCopy.mURLSpec)
 {
   // Whether or not an @import rule has a null sheet is a permanent
   // property of that @import rule, since it is null only if the target
   // sheet failed security checks.
   if (aCopy.mChildSheet) {
     RefPtr<StyleSheet> sheet =
       aCopy.mChildSheet->Clone(nullptr, this, nullptr, nullptr);
@@ -204,32 +203,24 @@ ImportRule::ImportRule(const ImportRule&
 
 ImportRule::~ImportRule()
 {
   if (mChildSheet) {
     mChildSheet->SetOwnerRule(nullptr);
   }
 }
 
-NS_IMPL_ADDREF_INHERITED(ImportRule, Rule)
-NS_IMPL_RELEASE_INHERITED(ImportRule, Rule)
-
-NS_IMPL_CYCLE_COLLECTION_INHERITED(ImportRule, Rule, mMedia, mChildSheet)
-
-bool
-ImportRule::IsCCLeaf() const
-{
-  // We're not a leaf.
-  return false;
-}
+NS_IMPL_ADDREF_INHERITED(ImportRule, CSSImportRule)
+NS_IMPL_RELEASE_INHERITED(ImportRule, CSSImportRule)
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(ImportRule, CSSImportRule, mMedia, mChildSheet)
 
 // QueryInterface implementation for ImportRule
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION_INHERITED(ImportRule)
-  NS_INTERFACE_MAP_ENTRY(nsIDOMCSSImportRule)
-NS_INTERFACE_MAP_END_INHERITING(Rule)
+NS_INTERFACE_MAP_END_INHERITING(CSSImportRule)
 
 #ifdef DEBUG
 /* virtual */ void
 ImportRule::List(FILE* out, int32_t aIndent) const
 {
   nsAutoCString str;
   // Indent
   for (int32_t indent = aIndent; --indent >= 0; ) {
@@ -243,22 +234,16 @@ ImportRule::List(FILE* out, int32_t aInd
   nsAutoString mediaText;
   mMedia->GetText(mediaText);
   AppendUTF16toUTF8(mediaText, str);
   str.AppendLiteral("\n");
   fprintf_stderr(out, "%s", str.get());
 }
 #endif
 
-/* virtual */ int32_t
-ImportRule::GetType() const
-{
-  return Rule::IMPORT_RULE;
-}
-
 /* virtual */ already_AddRefed<Rule>
 ImportRule::Clone() const
 {
   RefPtr<Rule> clone = new ImportRule(*this);
   return clone.forget();
 }
 
 void
@@ -269,22 +254,16 @@ ImportRule::SetSheet(CSSStyleSheet* aShe
   // set the new sheet
   mChildSheet = aSheet;
   aSheet->SetOwnerRule(this);
 
   // set our medialist to be the same as the sheet's medialist
   mMedia = static_cast<nsMediaList*>(mChildSheet->Media());
 }
 
-uint16_t
-ImportRule::Type() const
-{
-  return nsIDOMCSSRule::IMPORT_RULE;
-}
-
 void
 ImportRule::GetCssTextImpl(nsAString& aCssText) const
 {
   aCssText.AssignLiteral("@import url(");
   nsStyleUtil::AppendEscapedCSSString(mURLSpec, aCssText);
   aCssText.Append(')');
   if (mMedia) {
     nsAutoString mediaText;
@@ -311,55 +290,30 @@ ImportRule::GetStyleSheet() const
 
 NS_IMETHODIMP
 ImportRule::GetHref(nsAString & aHref)
 {
   aHref = mURLSpec;
   return NS_OK;
 }
 
-NS_IMETHODIMP
-ImportRule::GetMedia(nsIDOMMediaList * *aMedia)
-{
-  NS_ENSURE_ARG_POINTER(aMedia);
-
-  NS_ADDREF(*aMedia = mMedia);
-  return NS_OK;
-}
-
-NS_IMETHODIMP
-ImportRule::GetStyleSheet(nsIDOMCSSStyleSheet * *aStyleSheet)
-{
-  NS_ENSURE_ARG_POINTER(aStyleSheet);
-
-  NS_IF_ADDREF(*aStyleSheet = mChildSheet);
-  return NS_OK;
-}
-
 /* virtual */ size_t
 ImportRule::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
 {
   return aMallocSizeOf(this);
 
   // Measurement of the following members may be added later if DMD finds it is
   // worthwhile:
   // - mURLSpec
   //
   // The following members are not measured:
   // - mMedia, because it is measured via CSSStyleSheet::mMedia
   // - mChildSheet, because it is measured via CSSStyleSheetInner::mSheets
 }
 
-/* virtual */ JSObject*
-ImportRule::WrapObject(JSContext* aCx,
-                       JS::Handle<JSObject*> aGivenProto)
-{
-  return CSSImportRuleBinding::Wrap(aCx, this, aGivenProto);
-}
-
 // -------------------------------------------
 // nsICSSMediaRule
 //
 MediaRule::MediaRule(uint32_t aLineNumber, uint32_t aColumnNumber)
   : CSSMediaRule(aLineNumber, aColumnNumber)
 {
 }