--- a/layout/style/StyleRule.cpp
+++ b/layout/style/StyleRule.cpp
@@ -6,16 +6,17 @@
 /*
  * representation of CSS style rules (selectors+declaration), CSS
  * selectors, and DOM objects for style rules, selectors, and
  * declarations
  */
 
 #include "mozilla/css/StyleRule.h"
 
+#include "mozilla/DeclarationBlockInlines.h"
 #include "mozilla/StyleSheetInlines.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/css/GroupRule.h"
 #include "mozilla/css/Declaration.h"
 #include "nsIDocument.h"
 #include "nsIAtom.h"
 #include "nsString.h"
 #include "nsStyleUtil.h"
@@ -1057,18 +1058,18 @@ class DOMCSSDeclarationImpl : public nsD
 protected:
   virtual ~DOMCSSDeclarationImpl(void);
 
 public:
   explicit DOMCSSDeclarationImpl(css::StyleRule *aRule);
 
   NS_IMETHOD GetParentRule(nsIDOMCSSRule **aParent) override;
   void DropReference(void);
-  virtual css::Declaration* GetCSSDeclaration(Operation aOperation) override;
-  virtual nsresult SetCSSDeclaration(css::Declaration* aDecl) override;
+  virtual DeclarationBlock* GetCSSDeclaration(Operation aOperation) override;
+  virtual nsresult SetCSSDeclaration(DeclarationBlock* aDecl) override;
   virtual void GetCSSParsingEnvironment(CSSParsingEnvironment& aCSSParseEnv) override;
   virtual nsIDocument* DocToUpdate() override;
 
   // Override |AddRef| and |Release| for being a member of
   // |DOMCSSStyleRule|.  Also, we need to forward QI for cycle
   // collection things to DOMCSSStyleRule.
   NS_DECL_ISUPPORTS_INHERITED
 
@@ -1161,17 +1162,17 @@ NS_INTERFACE_MAP_BEGIN(DOMCSSDeclaration
 NS_IMPL_QUERY_TAIL_INHERITING(nsDOMCSSDeclaration)
 
 void
 DOMCSSDeclarationImpl::DropReference(void)
 {
   mRule = nullptr;
 }
 
-css::Declaration*
+DeclarationBlock*
 DOMCSSDeclarationImpl::GetCSSDeclaration(Operation aOperation)
 {
   if (mRule) {
     if (aOperation != eOperation_Read) {
       RefPtr<CSSStyleSheet> sheet = mRule->GetStyleSheet();
       if (sheet) {
         sheet->WillDirty();
       }
@@ -1198,30 +1199,30 @@ DOMCSSDeclarationImpl::GetParentRule(nsI
     return NS_OK;
   }
 
   NS_IF_ADDREF(*aParent = mRule->GetDOMRule());
   return NS_OK;
 }
 
 nsresult
-DOMCSSDeclarationImpl::SetCSSDeclaration(css::Declaration* aDecl)
+DOMCSSDeclarationImpl::SetCSSDeclaration(DeclarationBlock* aDecl)
 {
   NS_PRECONDITION(mRule,
          "can only be called when |GetCSSDeclaration| returned a declaration");
 
   nsCOMPtr<nsIDocument> owningDoc;
   RefPtr<CSSStyleSheet> sheet = mRule->GetStyleSheet();
   if (sheet) {
     owningDoc = sheet->GetOwningDocument();
   }
 
   mozAutoDocUpdate updateBatch(owningDoc, UPDATE_STYLE, true);
 
-  mRule->SetDeclaration(aDecl);
+  mRule->SetDeclaration(aDecl->AsGecko());
 
   if (sheet) {
     sheet->DidDirty();
   }
 
   if (owningDoc) {
     owningDoc->StyleRuleChanged(sheet, mRule);
   }

--- a/layout/style/nsCSSRules.cpp
+++ b/layout/style/nsCSSRules.cpp
@@ -24,17 +24,17 @@
 #include "nsIMediaList.h"
 #include "mozilla/dom/CSSRuleList.h"
 #include "nsIDocument.h"
 #include "nsPresContext.h"
 
 #include "nsContentUtils.h"
 #include "nsError.h"
 #include "nsStyleUtil.h"
-#include "mozilla/css/Declaration.h"
+#include "mozilla/DeclarationBlockInlines.h"
 #include "nsCSSParser.h"
 #include "nsDOMClassInfoID.h"
 #include "mozilla/dom/CSSStyleDeclarationBinding.h"
 #include "StyleRule.h"
 #include "nsFont.h"
 #include "nsIURI.h"
 #include "mozAutoDocUpdate.h"
 
@@ -1918,17 +1918,17 @@ NS_IMPL_CYCLE_COLLECTING_ADDREF(nsCSSKey
 NS_IMPL_CYCLE_COLLECTING_RELEASE(nsCSSKeyframeStyleDeclaration)
 
 NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_0(nsCSSKeyframeStyleDeclaration)
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsCSSKeyframeStyleDeclaration)
   NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
 NS_INTERFACE_MAP_END_INHERITING(nsDOMCSSDeclaration)
 
-css::Declaration*
+DeclarationBlock*
 nsCSSKeyframeStyleDeclaration::GetCSSDeclaration(Operation aOperation)
 {
   if (mRule) {
     return mRule->Declaration();
   } else {
     return nullptr;
   }
 }
@@ -1944,20 +1944,20 @@ nsCSSKeyframeStyleDeclaration::GetParent
 {
   NS_ENSURE_ARG_POINTER(aParent);
 
   NS_IF_ADDREF(*aParent = mRule);
   return NS_OK;
 }
 
 nsresult
-nsCSSKeyframeStyleDeclaration::SetCSSDeclaration(css::Declaration* aDecl)
+nsCSSKeyframeStyleDeclaration::SetCSSDeclaration(DeclarationBlock* aDecl)
 {
   MOZ_ASSERT(aDecl, "must be non-null");
-  mRule->ChangeDeclaration(aDecl);
+  mRule->ChangeDeclaration(aDecl->AsGecko());
   return NS_OK;
 }
 
 nsIDocument*
 nsCSSKeyframeStyleDeclaration::DocToUpdate()
 {
   return nullptr;
 }
@@ -2465,17 +2465,17 @@ NS_IMPL_CYCLE_COLLECTING_ADDREF(nsCSSPag
 NS_IMPL_CYCLE_COLLECTING_RELEASE(nsCSSPageStyleDeclaration)
 
 NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_0(nsCSSPageStyleDeclaration)
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsCSSPageStyleDeclaration)
   NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
 NS_INTERFACE_MAP_END_INHERITING(nsDOMCSSDeclaration)
 
-css::Declaration*
+DeclarationBlock*
 nsCSSPageStyleDeclaration::GetCSSDeclaration(Operation aOperation)
 {
   if (mRule) {
     return mRule->Declaration();
   } else {
     return nullptr;
   }
 }
@@ -2491,20 +2491,20 @@ nsCSSPageStyleDeclaration::GetParentRule
 {
   NS_ENSURE_ARG_POINTER(aParent);
 
   NS_IF_ADDREF(*aParent = mRule);
   return NS_OK;
 }
 
 nsresult
-nsCSSPageStyleDeclaration::SetCSSDeclaration(css::Declaration* aDecl)
+nsCSSPageStyleDeclaration::SetCSSDeclaration(DeclarationBlock* aDecl)
 {
   MOZ_ASSERT(aDecl, "must be non-null");
-  mRule->ChangeDeclaration(aDecl);
+  mRule->ChangeDeclaration(aDecl->AsGecko());
   return NS_OK;
 }
 
 nsIDocument*
 nsCSSPageStyleDeclaration::DocToUpdate()
 {
   return nullptr;
 }

--- a/layout/style/nsCSSRules.h
+++ b/layout/style/nsCSSRules.h
@@ -350,18 +350,18 @@ class nsCSSKeyframeRule;
 
 class nsCSSKeyframeStyleDeclaration final : public nsDOMCSSDeclaration
 {
 public:
   explicit nsCSSKeyframeStyleDeclaration(nsCSSKeyframeRule *aRule);
 
   NS_IMETHOD GetParentRule(nsIDOMCSSRule **aParent) override;
   void DropReference() { mRule = nullptr; }
-  virtual mozilla::css::Declaration* GetCSSDeclaration(Operation aOperation) override;
-  virtual nsresult SetCSSDeclaration(mozilla::css::Declaration* aDecl) override;
+  virtual mozilla::DeclarationBlock* GetCSSDeclaration(Operation aOperation) override;
+  virtual nsresult SetCSSDeclaration(mozilla::DeclarationBlock* aDecl) override;
   virtual void GetCSSParsingEnvironment(CSSParsingEnvironment& aCSSParseEnv) override;
   virtual nsIDocument* DocToUpdate() override;
 
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
   NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_AMBIGUOUS(nsCSSKeyframeStyleDeclaration,
                                                          nsICSSDeclaration)
 
   virtual nsINode* GetParentObject() override;
@@ -480,18 +480,18 @@ class nsCSSPageRule;
 
 class nsCSSPageStyleDeclaration final : public nsDOMCSSDeclaration
 {
 public:
   explicit nsCSSPageStyleDeclaration(nsCSSPageRule *aRule);
 
   NS_IMETHOD GetParentRule(nsIDOMCSSRule **aParent) override;
   void DropReference() { mRule = nullptr; }
-  virtual mozilla::css::Declaration* GetCSSDeclaration(Operation aOperation) override;
-  virtual nsresult SetCSSDeclaration(mozilla::css::Declaration* aDecl) override;
+  virtual mozilla::DeclarationBlock* GetCSSDeclaration(Operation aOperation) override;
+  virtual nsresult SetCSSDeclaration(mozilla::DeclarationBlock* aDecl) override;
   virtual void GetCSSParsingEnvironment(CSSParsingEnvironment& aCSSParseEnv) override;
   virtual nsIDocument* DocToUpdate() override;
 
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
   NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_AMBIGUOUS(nsCSSPageStyleDeclaration,
                                                          nsICSSDeclaration)
 
   virtual nsINode *GetParentObject() override;

--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -554,25 +554,25 @@ nsComputedDOMStyle::GetPresShellForConte
     return nullptr;
 
   return composedDoc->GetShell();
 }
 
 // nsDOMCSSDeclaration abstract methods which should never be called
 // on a nsComputedDOMStyle object, but must be defined to avoid
 // compile errors.
-css::Declaration*
+DeclarationBlock*
 nsComputedDOMStyle::GetCSSDeclaration(Operation)
 {
   NS_RUNTIMEABORT("called nsComputedDOMStyle::GetCSSDeclaration");
   return nullptr;
 }
 
 nsresult
-nsComputedDOMStyle::SetCSSDeclaration(css::Declaration*)
+nsComputedDOMStyle::SetCSSDeclaration(DeclarationBlock*)
 {
   NS_RUNTIMEABORT("called nsComputedDOMStyle::SetCSSDeclaration");
   return NS_ERROR_FAILURE;
 }
 
 nsIDocument*
 nsComputedDOMStyle::DocToUpdate()
 {

--- a/layout/style/nsComputedDOMStyle.h
+++ b/layout/style/nsComputedDOMStyle.h
@@ -100,18 +100,18 @@ public:
   void SetExposeVisitedStyle(bool aExpose) {
     NS_ASSERTION(aExpose != mExposeVisitedStyle, "should always be changing");
     mExposeVisitedStyle = aExpose;
   }
 
   // nsDOMCSSDeclaration abstract methods which should never be called
   // on a nsComputedDOMStyle object, but must be defined to avoid
   // compile errors.
-  virtual mozilla::css::Declaration* GetCSSDeclaration(Operation) override;
-  virtual nsresult SetCSSDeclaration(mozilla::css::Declaration*) override;
+  virtual mozilla::DeclarationBlock* GetCSSDeclaration(Operation) override;
+  virtual nsresult SetCSSDeclaration(mozilla::DeclarationBlock*) override;
   virtual nsIDocument* DocToUpdate() override;
   virtual void GetCSSParsingEnvironment(CSSParsingEnvironment& aCSSParseEnv) override;
 
   static already_AddRefed<nsROCSSPrimitiveValue>
     MatrixToCSSValue(const mozilla::gfx::Matrix4x4& aMatrix);
 
   static void RegisterPrefChangeCallbacks();
   static void UnregisterPrefChangeCallbacks();

--- a/layout/style/nsDOMCSSAttrDeclaration.cpp
+++ b/layout/style/nsDOMCSSAttrDeclaration.cpp
@@ -64,44 +64,43 @@ NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_THIS_E
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsDOMCSSAttributeDeclaration)
   NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
 NS_IMPL_QUERY_TAIL_INHERITING(nsDOMCSSDeclaration)
 
 NS_IMPL_CYCLE_COLLECTING_ADDREF(nsDOMCSSAttributeDeclaration)
 NS_IMPL_CYCLE_COLLECTING_RELEASE(nsDOMCSSAttributeDeclaration)
 
 nsresult
-nsDOMCSSAttributeDeclaration::SetCSSDeclaration(css::Declaration* aDecl)
+nsDOMCSSAttributeDeclaration::SetCSSDeclaration(DeclarationBlock* aDecl)
 {
   NS_ASSERTION(mElement, "Must have Element to set the declaration!");
-  return
-    mIsSMILOverride ? mElement->SetSMILOverrideStyleDeclaration(aDecl, true) :
-    mElement->SetInlineStyleDeclaration(aDecl, nullptr, true);
+  return mIsSMILOverride
+    ? mElement->SetSMILOverrideStyleDeclaration(aDecl->AsGecko(), true)
+    : mElement->SetInlineStyleDeclaration(aDecl, nullptr, true);
 }
 
 nsIDocument*
 nsDOMCSSAttributeDeclaration::DocToUpdate()
 {
   // We need OwnerDoc() rather than GetUncomposedDoc() because it might
   // be the BeginUpdate call that inserts mElement into the document.
   return mElement->OwnerDoc();
 }
 
-css::Declaration*
+DeclarationBlock*
 nsDOMCSSAttributeDeclaration::GetCSSDeclaration(Operation aOperation)
 {
   if (!mElement)
     return nullptr;
 
-  css::Declaration* declaration;
+  DeclarationBlock* declaration;
   if (mIsSMILOverride) {
     declaration = mElement->GetSMILOverrideStyleDeclaration();
   } else {
-    DeclarationBlock* decl = mElement->GetInlineStyleDeclaration();
-    declaration = decl && decl->IsGecko() ? decl->AsGecko() : nullptr;
+    declaration = mElement->GetInlineStyleDeclaration();
   }
 
   // Notify observers that our style="" attribute is going to change
   // unless:
   //   * this is a declaration that holds SMIL animation values (which
   //     aren't reflected in the DOM style="" attribute), or
   //   * we're getting the declaration for reading, or
   //   * we're getting it for property removal but we don't currently have
@@ -129,20 +128,21 @@ nsDOMCSSAttributeDeclaration::GetCSSDecl
   }
 
   // cannot fail
   RefPtr<css::Declaration> decl = new css::Declaration();
   decl->InitializeEmpty();
 
   // this *can* fail (inside SetAttrAndNotify, at least).
   nsresult rv;
-  if (mIsSMILOverride)
+  if (mIsSMILOverride) {
     rv = mElement->SetSMILOverrideStyleDeclaration(decl, false);
-  else
+  } else {
     rv = mElement->SetInlineStyleDeclaration(decl, nullptr, false);
+  }
 
   if (NS_FAILED(rv)) {
     return nullptr; // the decl will be destroyed along with the style rule
   }
 
   return decl;
 }
 

--- a/layout/style/nsDOMCSSAttrDeclaration.h
+++ b/layout/style/nsDOMCSSAttrDeclaration.h
@@ -25,29 +25,29 @@ public:
   nsDOMCSSAttributeDeclaration(Element* aContent, bool aIsSMILOverride);
 
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
   NS_DECL_CYCLE_COLLECTION_SKIPPABLE_SCRIPT_HOLDER_CLASS_AMBIGUOUS(nsDOMCSSAttributeDeclaration,
                                                                    nsICSSDeclaration)
 
   // If GetCSSDeclaration returns non-null, then the decl it returns
   // is owned by our current style rule.
-  virtual mozilla::css::Declaration* GetCSSDeclaration(Operation aOperation) override;
+  virtual mozilla::DeclarationBlock* GetCSSDeclaration(Operation aOperation) override;
   virtual void GetCSSParsingEnvironment(CSSParsingEnvironment& aCSSParseEnv) override;
   NS_IMETHOD GetParentRule(nsIDOMCSSRule **aParent) override;
 
   virtual nsINode* GetParentObject() override;
 
   NS_IMETHOD SetPropertyValue(const nsCSSPropertyID aPropID,
                               const nsAString& aValue) override;
 
 protected:
   ~nsDOMCSSAttributeDeclaration();
 
-  virtual nsresult SetCSSDeclaration(mozilla::css::Declaration* aDecl) override;
+  virtual nsresult SetCSSDeclaration(mozilla::DeclarationBlock* aDecl) override;
   virtual nsIDocument* DocToUpdate() override;
 
   RefPtr<Element> mElement;
 
   /* If true, this indicates that this nsDOMCSSAttributeDeclaration
    * should interact with mContent's SMIL override style rule (rather
    * than the inline style rule).
    */

--- a/layout/style/nsDOMCSSDeclaration.cpp
+++ b/layout/style/nsDOMCSSDeclaration.cpp
@@ -3,16 +3,17 @@
  * 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/. */
 
 /* base class for DOM objects for element.style and cssStyleRule.style */
 
 #include "nsDOMCSSDeclaration.h"
 
 #include "nsCSSParser.h"
+#include "mozilla/DeclarationBlockInlines.h"
 #include "mozilla/StyleSheetInlines.h"
 #include "mozilla/css/Rule.h"
 #include "mozilla/DeclarationBlockInlines.h"
 #include "mozilla/dom/CSS2PropertiesBinding.h"
 #include "nsCSSProps.h"
 #include "nsCOMPtr.h"
 #include "mozAutoDocUpdate.h"
 #include "nsIURI.h"
@@ -42,33 +43,33 @@ NS_INTERFACE_MAP_END
 
 NS_IMETHODIMP
 nsDOMCSSDeclaration::GetPropertyValue(const nsCSSPropertyID aPropID,
                                       nsAString& aValue)
 {
   NS_PRECONDITION(aPropID != eCSSProperty_UNKNOWN,
                   "Should never pass eCSSProperty_UNKNOWN around");
 
-  css::Declaration* decl = GetCSSDeclaration(eOperation_Read);
+  css::Declaration* decl = GetCSSDeclaration(eOperation_Read)->AsGecko();
 
   aValue.Truncate();
   if (decl) {
     decl->GetValue(aPropID, aValue);
   }
   return NS_OK;
 }
 
 void
 nsDOMCSSDeclaration::GetCustomPropertyValue(const nsAString& aPropertyName,
                                             nsAString& aValue)
 {
   MOZ_ASSERT(Substring(aPropertyName, 0,
                        CSS_CUSTOM_NAME_PREFIX_LENGTH).EqualsLiteral("--"));
 
-  css::Declaration* decl = GetCSSDeclaration(eOperation_Read);
+  css::Declaration* decl = GetCSSDeclaration(eOperation_Read)->AsGecko();
   if (!decl) {
     aValue.Truncate();
     return;
   }
 
   decl->GetVariableDeclaration(Substring(aPropertyName,
                                          CSS_CUSTOM_NAME_PREFIX_LENGTH),
                                aValue);
@@ -110,32 +111,32 @@ nsDOMCSSDeclaration::SetPropertyValue(co
 
   return ParsePropertyValue(aPropID, aValue, false);
 }
 
 
 NS_IMETHODIMP
 nsDOMCSSDeclaration::GetCssText(nsAString& aCssText)
 {
-  css::Declaration* decl = GetCSSDeclaration(eOperation_Read);
+  css::Declaration* decl = GetCSSDeclaration(eOperation_Read)->AsGecko();
   aCssText.Truncate();
 
   if (decl) {
     decl->ToString(aCssText);
   }
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsDOMCSSDeclaration::SetCssText(const nsAString& aCssText)
 {
   // We don't need to *do* anything with the old declaration, but we need
   // to ensure that it exists, or else SetCSSDeclaration may crash.
-  css::Declaration* olddecl = GetCSSDeclaration(eOperation_Modify);
+  css::Declaration* olddecl = GetCSSDeclaration(eOperation_Modify)->AsGecko();
   if (!olddecl) {
     return NS_ERROR_NOT_AVAILABLE;
   }
 
   CSSParsingEnvironment env;
   GetCSSParsingEnvironment(env);
   if (!env.mPrincipal) {
     return NS_ERROR_NOT_AVAILABLE;
@@ -160,17 +161,17 @@ nsDOMCSSDeclaration::SetCssText(const ns
   }
 
   return SetCSSDeclaration(decl);
 }
 
 NS_IMETHODIMP
 nsDOMCSSDeclaration::GetLength(uint32_t* aLength)
 {
-  css::Declaration* decl = GetCSSDeclaration(eOperation_Read);
+  css::Declaration* decl = GetCSSDeclaration(eOperation_Read)->AsGecko();
 
   if (decl) {
     *aLength = decl->Count();
   } else {
     *aLength = 0;
   }
 
   return NS_OK;
@@ -182,17 +183,17 @@ nsDOMCSSDeclaration::GetPropertyCSSValue
   // We don't support CSSValue yet so we'll just return null...
 
   return nullptr;
 }
 
 void
 nsDOMCSSDeclaration::IndexedGetter(uint32_t aIndex, bool& aFound, nsAString& aPropName)
 {
-  css::Declaration* decl = GetCSSDeclaration(eOperation_Read);
+  css::Declaration* decl = GetCSSDeclaration(eOperation_Read)->AsGecko();
   aFound = decl && decl->GetNthProperty(aIndex, aPropName);
 }
 
 NS_IMETHODIMP
 nsDOMCSSDeclaration::GetPropertyValue(const nsAString& aPropertyName,
                                       nsAString& aReturn)
 {
   const nsCSSPropertyID propID =
@@ -221,30 +222,30 @@ nsDOMCSSDeclaration::GetAuthoredProperty
     return NS_OK;
   }
 
   if (propID == eCSSPropertyExtra_variable) {
     GetCustomPropertyValue(aPropertyName, aReturn);
     return NS_OK;
   }
 
-  css::Declaration* decl = GetCSSDeclaration(eOperation_Read);
+  css::Declaration* decl = GetCSSDeclaration(eOperation_Read)->AsGecko();
   if (!decl) {
     return NS_ERROR_FAILURE;
   }
 
   decl->GetAuthoredValue(propID, aReturn);
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsDOMCSSDeclaration::GetPropertyPriority(const nsAString& aPropertyName,
                                          nsAString& aReturn)
 {
-  css::Declaration* decl = GetCSSDeclaration(eOperation_Read);
+  css::Declaration* decl = GetCSSDeclaration(eOperation_Read)->AsGecko();
 
   aReturn.Truncate();
   if (decl && decl->GetValueIsImportant(aPropertyName)) {
     aReturn.AssignLiteral("important");
   }
 
   return NS_OK;
 }
@@ -326,17 +327,17 @@ nsDOMCSSDeclaration::GetCSSParsingEnviro
   aCSSParseEnv.mCSSLoader = document ? document->CSSLoader() : nullptr;
 }
 
 nsresult
 nsDOMCSSDeclaration::ParsePropertyValue(const nsCSSPropertyID aPropID,
                                         const nsAString& aPropValue,
                                         bool aIsImportant)
 {
-  css::Declaration* olddecl = GetCSSDeclaration(eOperation_Modify);
+  css::Declaration* olddecl = GetCSSDeclaration(eOperation_Modify)->AsGecko();
   if (!olddecl) {
     return NS_ERROR_NOT_AVAILABLE;
   }
 
   CSSParsingEnvironment env;
   GetCSSParsingEnvironment(env);
   if (!env.mPrincipal) {
     return NS_ERROR_NOT_AVAILABLE;
@@ -364,17 +365,17 @@ nsDOMCSSDeclaration::ParsePropertyValue(
 
 nsresult
 nsDOMCSSDeclaration::ParseCustomPropertyValue(const nsAString& aPropertyName,
                                               const nsAString& aPropValue,
                                               bool aIsImportant)
 {
   MOZ_ASSERT(nsCSSProps::IsCustomPropertyName(aPropertyName));
 
-  css::Declaration* olddecl = GetCSSDeclaration(eOperation_Modify);
+  css::Declaration* olddecl = GetCSSDeclaration(eOperation_Modify)->AsGecko();
   if (!olddecl) {
     return NS_ERROR_NOT_AVAILABLE;
   }
 
   CSSParsingEnvironment env;
   GetCSSParsingEnvironment(env);
   if (!env.mPrincipal) {
     return NS_ERROR_NOT_AVAILABLE;
@@ -401,17 +402,18 @@ nsDOMCSSDeclaration::ParseCustomProperty
   }
 
   return SetCSSDeclaration(decl);
 }
 
 nsresult
 nsDOMCSSDeclaration::RemoveProperty(const nsCSSPropertyID aPropID)
 {
-  css::Declaration* olddecl = GetCSSDeclaration(eOperation_RemoveProperty);
+  css::Declaration* olddecl =
+    GetCSSDeclaration(eOperation_RemoveProperty)->AsGecko();
   if (!olddecl) {
     return NS_OK; // no decl, so nothing to remove
   }
 
   // For nsDOMCSSAttributeDeclaration, SetCSSDeclaration will lead to
   // Attribute setting code, which leads in turn to BeginUpdate.  We
   // need to start the update now so that the old rule doesn't get used
   // between when we mutate the declaration and when we set the new
@@ -424,17 +426,18 @@ nsDOMCSSDeclaration::RemoveProperty(cons
 }
 
 nsresult
 nsDOMCSSDeclaration::RemoveCustomProperty(const nsAString& aPropertyName)
 {
   MOZ_ASSERT(Substring(aPropertyName, 0,
                        CSS_CUSTOM_NAME_PREFIX_LENGTH).EqualsLiteral("--"));
 
-  css::Declaration* olddecl = GetCSSDeclaration(eOperation_RemoveProperty);
+  css::Declaration* olddecl =
+    GetCSSDeclaration(eOperation_RemoveProperty)->AsGecko();
   if (!olddecl) {
     return NS_OK; // no decl, so nothing to remove
   }
 
   // For nsDOMCSSAttributeDeclaration, SetCSSDeclaration will lead to
   // Attribute setting code, which leads in turn to BeginUpdate.  We
   // need to start the update now so that the old rule doesn't get used
   // between when we mutate the declaration and when we set the new

--- a/layout/style/nsDOMCSSDeclaration.h
+++ b/layout/style/nsDOMCSSDeclaration.h
@@ -15,18 +15,18 @@
 #include "nsCOMPtr.h"
 
 class nsIPrincipal;
 class nsIDocument;
 struct JSContext;
 class JSObject;
 
 namespace mozilla {
+class DeclarationBlock;
 namespace css {
-class Declaration;
 class Loader;
 class Rule;
 } // namespace css
 } // namespace mozilla
 
 class nsDOMCSSDeclaration : public nsICSSDeclaration
 {
 public:
@@ -114,18 +114,18 @@ protected:
     eOperation_Modify,
 
     // We are calling GetCSSDeclaration so that we can remove a property from
     // it.  Does not allocates a new declaration if we don't have one yet;
     // returns nullptr in this case.  If we do have a declaration, calls
     // AttributeWillChange.
     eOperation_RemoveProperty
   };
-  virtual mozilla::css::Declaration* GetCSSDeclaration(Operation aOperation) = 0;
-  virtual nsresult SetCSSDeclaration(mozilla::css::Declaration* aDecl) = 0;
+  virtual mozilla::DeclarationBlock* GetCSSDeclaration(Operation aOperation) = 0;
+  virtual nsresult SetCSSDeclaration(mozilla::DeclarationBlock* aDecl) = 0;
   // Document that we must call BeginUpdate/EndUpdate on around the
   // calls to SetCSSDeclaration and the style rule mutation that leads
   // to it.
   virtual nsIDocument* DocToUpdate() = 0;
 
   // Information neded to parse a declaration.  We need the mSheetURI
   // for error reporting, mBaseURI to resolve relative URIs,
   // mPrincipal for subresource loads, and mCSSLoader for determining