diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index a377b06e47..7522128d46 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -56,7 +56,6 @@
 #include "mozilla/AnimationComparator.h"
 #include "mozilla/AsyncEventDispatcher.h"
 #include "mozilla/ContentEvents.h"
-#include "mozilla/DeclarationBlockInlines.h"
 #include "mozilla/EffectSet.h"
 #include "mozilla/EventDispatcher.h"
 #include "mozilla/EventListenerManager.h"
@@ -108,6 +107,7 @@
 #include "nsViewManager.h"
 #include "nsIScrollableFrame.h"
 #include "mozilla/css/StyleRule.h" /* For nsCSSSelectorList */
+#include "mozilla/css/Declaration.h"
 #include "nsCSSRuleProcessor.h"
 #include "nsRuleProcessorData.h"
 #include "nsTextNode.h"
@@ -2024,7 +2024,7 @@ Element::GetSMILOverrideStyle()
   return slots->mSMILOverrideStyle;
 }
 
-DeclarationBlock*
+css::Declaration*
 Element::GetSMILOverrideStyleDeclaration()
 {
   Element::nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
@@ -2032,7 +2032,7 @@ Element::GetSMILOverrideStyleDeclaration()
 }
 
 nsresult
-Element::SetSMILOverrideStyleDeclaration(DeclarationBlock* aDeclaration,
+Element::SetSMILOverrideStyleDeclaration(css::Declaration* aDeclaration,
                                          bool aNotify)
 {
   Element::nsExtendedDOMSlots* slots = ExtendedDOMSlots();
@@ -2072,14 +2072,14 @@ Element::IsInteractiveHTMLContent(bool aIgnoreTabindex) const
   return false;
 }
 
-DeclarationBlock*
+css::Declaration*
 Element::GetInlineStyleDeclaration()
 {
   return nullptr;
 }
 
 nsresult
-Element::SetInlineStyleDeclaration(DeclarationBlock* aDeclaration,
+Element::SetInlineStyleDeclaration(css::Declaration* aDeclaration,
                                    const nsAString* aSerialized,
                                    bool aNotify)
 {
diff --git a/dom/base/Element.h b/dom/base/Element.h
index cf342e94d1..4d4c9284d3 100644
--- a/dom/base/Element.h
+++ b/dom/base/Element.h
@@ -57,7 +57,9 @@ class nsDocument;
 class nsDOMStringMap;
 
 namespace mozilla {
-class DeclarationBlock;
+namespace css {
+  class Declaration;
+} // namespace css
 namespace dom {
   struct AnimationFilter;
   struct ScrollIntoViewOptions;
@@ -274,13 +276,13 @@ public:
   /**
    * Get the inline style declaration, if any, for this element.
    */
-  virtual DeclarationBlock* GetInlineStyleDeclaration();
+  virtual css::Declaration* GetInlineStyleDeclaration();
 
   /**
    * Set the inline style declaration for this element. This will send
    * an appropriate AttributeChanged notification if aNotify is true.
    */
-  virtual nsresult SetInlineStyleDeclaration(DeclarationBlock* aDeclaration,
+  virtual nsresult SetInlineStyleDeclaration(css::Declaration* aDeclaration,
                                              const nsAString* aSerialized,
                                              bool aNotify);
 
@@ -288,7 +290,7 @@ public:
    * Get the SMIL override style declaration for this element. If the
    * rule hasn't been created, this method simply returns null.
    */
-  virtual DeclarationBlock* GetSMILOverrideStyleDeclaration();
+  virtual css::Declaration* GetSMILOverrideStyleDeclaration();
 
   /**
    * Set the SMIL override style declaration for this element. If
@@ -296,7 +298,7 @@ public:
    * context, so that the style changes will be noticed.
    */
   virtual nsresult SetSMILOverrideStyleDeclaration(
-    DeclarationBlock* aDeclaration, bool aNotify);
+    css::Declaration* aDeclaration, bool aNotify);
 
   /**
    * Returns a new nsISMILAttr that allows the caller to animate the given
diff --git a/dom/base/FragmentOrElement.cpp b/dom/base/FragmentOrElement.cpp
index 126267fe93..1c2fc05b09 100644
--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -17,7 +17,6 @@
 #include "mozilla/dom/FragmentOrElement.h"
 
 #include "mozilla/AsyncEventDispatcher.h"
-#include "mozilla/DeclarationBlockInlines.h"
 #include "mozilla/EffectSet.h"
 #include "mozilla/EventDispatcher.h"
 #include "mozilla/EventListenerManager.h"
@@ -99,6 +98,7 @@
 #include "nsIScrollableFrame.h"
 #include "ChildIterator.h"
 #include "mozilla/css/StyleRule.h" /* For nsCSSSelectorList */
+#include "mozilla/css/Declaration.h"
 #include "nsRuleProcessorData.h"
 #include "nsTextNode.h"
 #include "mozilla/dom/NodeListBinding.h"
diff --git a/dom/base/FragmentOrElement.h b/dom/base/FragmentOrElement.h
index 2f65fe5174..50c10536a9 100644
--- a/dom/base/FragmentOrElement.h
+++ b/dom/base/FragmentOrElement.h
@@ -35,7 +35,9 @@ class nsDOMStringMap;
 class nsIURI;
 
 namespace mozilla {
-class DeclarationBlock;
+namespace css {
+class Declaration;
+}
 namespace dom {
 struct CustomElementData;
 class DOMIntersectionObserver;
@@ -265,7 +267,7 @@ public:
     /**
      * Holds any SMIL override style declaration for this element.
      */
-    RefPtr<mozilla::DeclarationBlock> mSMILOverrideStyleDeclaration;
+    RefPtr<mozilla::css::Declaration> mSMILOverrideStyleDeclaration;
 
     /**
     * The nearest enclosing content node with a binding that created us.
diff --git a/dom/base/nsAttrValue.cpp b/dom/base/nsAttrValue.cpp
index cdc95a570e..9c4dcc9236 100644
--- a/dom/base/nsAttrValue.cpp
+++ b/dom/base/nsAttrValue.cpp
@@ -16,7 +16,7 @@
 #include "nsIAtom.h"
 #include "nsUnicharUtils.h"
 #include "mozilla/MemoryReporting.h"
-#include "mozilla/DeclarationBlockInlines.h"
+#include "mozilla/css/Declaration.h"
 #include "nsContentUtils.h"
 #include "nsReadableUtils.h"
 #include "prprf.h"
@@ -149,7 +149,7 @@ nsAttrValue::nsAttrValue(nsIAtom* aValue)
   SetTo(aValue);
 }
 
-nsAttrValue::nsAttrValue(already_AddRefed<DeclarationBlock> aValue,
+nsAttrValue::nsAttrValue(already_AddRefed<css::Declaration> aValue,
                          const nsAString* aSerialized)
     : mBits(0)
 {
@@ -419,7 +419,7 @@ nsAttrValue::SetTo(double aValue, const nsAString* aSerialized)
 }
 
 void
-nsAttrValue::SetTo(already_AddRefed<DeclarationBlock> aValue,
+nsAttrValue::SetTo(already_AddRefed<css::Declaration> aValue,
                    const nsAString* aSerialized)
 {
   MiscContainer* cont = EnsureEmptyMiscContainer();
@@ -642,7 +642,7 @@ nsAttrValue::ToString(nsAString& aResult) const
     {
       aResult.Truncate();
       MiscContainer *container = GetMiscContainer();
-      if (DeclarationBlock* decl = container->mValue.mCSSDeclaration) {
+      if (css::Declaration* decl = container->mValue.mCSSDeclaration) {
         decl->ToString(aResult);
       }
       const_cast<nsAttrValue*>(this)->SetMiscAtomOrString(&aResult);
@@ -1747,7 +1747,7 @@ nsAttrValue::ParseStyleAttribute(const nsAString& aString,
     }
   }
 
-  RefPtr<DeclarationBlock> decl;
+  RefPtr<css::Declaration> decl;
   css::Loader* cssLoader = ownerDoc->CSSLoader();
   nsCSSParser cssParser(cssLoader);
   decl = cssParser.ParseStyleAttribute(aString, docURI, baseURI,
diff --git a/dom/base/nsAttrValue.h b/dom/base/nsAttrValue.h
index f2cb17aebf..831633afa2 100644
--- a/dom/base/nsAttrValue.h
+++ b/dom/base/nsAttrValue.h
@@ -36,8 +36,8 @@ class nsStyledElement;
 struct MiscContainer;
 
 namespace mozilla {
-class DeclarationBlock;
 namespace css {
+class Declaration;
 struct URLValue;
 struct ImageValue;
 } // namespace css
@@ -122,7 +122,7 @@ public:
   nsAttrValue(const nsAttrValue& aOther);
   explicit nsAttrValue(const nsAString& aValue);
   explicit nsAttrValue(nsIAtom* aValue);
-  nsAttrValue(already_AddRefed<mozilla::DeclarationBlock> aValue,
+  nsAttrValue(already_AddRefed<mozilla::css::Declaration> aValue,
               const nsAString* aSerialized);
   explicit nsAttrValue(const nsIntMargin& aValue);
   ~nsAttrValue();
@@ -148,7 +148,7 @@ public:
   void SetTo(int16_t aInt);
   void SetTo(int32_t aInt, const nsAString* aSerialized);
   void SetTo(double aValue, const nsAString* aSerialized);
-  void SetTo(already_AddRefed<mozilla::DeclarationBlock> aValue,
+  void SetTo(already_AddRefed<mozilla::css::Declaration> aValue,
              const nsAString* aSerialized);
   void SetTo(mozilla::css::URLValue* aValue, const nsAString* aSerialized);
   void SetTo(const nsIntMargin& aValue);
@@ -200,7 +200,7 @@ public:
   inline int16_t GetEnumValue() const;
   inline float GetPercentValue() const;
   inline AtomArray* GetAtomArrayValue() const;
-  inline mozilla::DeclarationBlock* GetCSSDeclarationValue() const;
+  inline mozilla::css::Declaration* GetCSSDeclarationValue() const;
   inline mozilla::css::URLValue* GetURLValue() const;
   inline mozilla::css::ImageValue* GetImageValue() const;
   inline double GetDoubleValue() const;
diff --git a/dom/base/nsAttrValueInlines.h b/dom/base/nsAttrValueInlines.h
index 382270f157..8909111137 100644
--- a/dom/base/nsAttrValueInlines.h
+++ b/dom/base/nsAttrValueInlines.h
@@ -30,7 +30,7 @@ struct MiscContainer final
         nscolor mColor;
         uint32_t mEnumValue;
         int32_t mPercent;
-        mozilla::DeclarationBlock* mCSSDeclaration;
+        mozilla::css::Declaration* mCSSDeclaration;
         mozilla::css::URLValue* mURL;
         mozilla::css::ImageValue* mImage;
         nsAttrValue::AtomArray* mAtomArray;
@@ -145,7 +145,7 @@ nsAttrValue::GetAtomArrayValue() const
   return GetMiscContainer()->mValue.mAtomArray;
 }
 
-inline mozilla::DeclarationBlock*
+inline mozilla::css::Declaration*
 nsAttrValue::GetCSSDeclarationValue() const
 {
   NS_PRECONDITION(Type() == eCSSDeclaration, "wrong type");
diff --git a/dom/base/nsStyledElement.cpp b/dom/base/nsStyledElement.cpp
index b9331c076d..607926a484 100644
--- a/dom/base/nsStyledElement.cpp
+++ b/dom/base/nsStyledElement.cpp
@@ -14,9 +14,9 @@
 #include "nsDOMCSSAttrDeclaration.h"
 #include "nsServiceManagerUtils.h"
 #include "nsIDocument.h"
-#include "mozilla/DeclarationBlockInlines.h"
 #include "nsCSSParser.h"
 #include "mozilla/css/Loader.h"
+#include "mozilla/css/Declaration.h"
 #include "nsIDOMMutationEvent.h"
 #include "nsXULElement.h"
 #include "nsContentUtils.h"
@@ -64,7 +64,7 @@ nsStyledElement::BeforeSetAttr(int32_t aNamespaceID, nsIAtom* aName,
 }
 
 nsresult
-nsStyledElement::SetInlineStyleDeclaration(DeclarationBlock* aDeclaration,
+nsStyledElement::SetInlineStyleDeclaration(css::Declaration* aDeclaration,
                                            const nsAString* aSerialized,
                                            bool aNotify)
 {
@@ -113,7 +113,7 @@ nsStyledElement::SetInlineStyleDeclaration(DeclarationBlock* aDeclaration,
                           document, updateBatch);
 }
 
-DeclarationBlock*
+css::Declaration*
 nsStyledElement::GetInlineStyleDeclaration()
 {
   if (!MayHaveStyle()) {
diff --git a/dom/base/nsStyledElement.h b/dom/base/nsStyledElement.h
index 4c32a049b2..38e76f44ae 100644
--- a/dom/base/nsStyledElement.h
+++ b/dom/base/nsStyledElement.h
@@ -17,7 +17,9 @@
 #include "mozilla/dom/Element.h"
 
 namespace mozilla {
-class DeclarationBlock;
+namespace css {
+class Declaration;
+} // namespace css
 } // namespace mozilla
 
 // IID for nsStyledElement interface
@@ -43,8 +45,8 @@ public:
   NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) override;
 
   // Element interface methods
-  virtual mozilla::DeclarationBlock* GetInlineStyleDeclaration() override;
-  virtual nsresult SetInlineStyleDeclaration(mozilla::DeclarationBlock* aDeclaration,
+  virtual mozilla::css::Declaration* GetInlineStyleDeclaration() override;
+  virtual nsresult SetInlineStyleDeclaration(mozilla::css::Declaration* aDeclaration,
                                              const nsAString* aSerialized,
                                              bool aNotify) override;
 
diff --git a/dom/html/nsGenericHTMLElement.cpp b/dom/html/nsGenericHTMLElement.cpp
index da7121a714..b34f10df6e 100644
--- a/dom/html/nsGenericHTMLElement.cpp
+++ b/dom/html/nsGenericHTMLElement.cpp
@@ -4,7 +4,6 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/ArrayUtils.h"
-#include "mozilla/DeclarationBlockInlines.h"
 #include "mozilla/EventDispatcher.h"
 #include "mozilla/EventListenerManager.h"
 #include "mozilla/EventStateManager.h"
@@ -189,11 +188,11 @@ nsGenericHTMLElement::CopyInnerTo(Element* aDst)
 
     if (name->Equals(nsGkAtoms::style, kNameSpaceID_None) &&
         value->Type() == nsAttrValue::eCSSDeclaration) {
-      DeclarationBlock* decl = value->GetCSSDeclarationValue();
+      css::Declaration* decl = value->GetCSSDeclarationValue();
       // We can't just set this as a string, because that will fail
       // to reparse the string into style data until the node is
       // inserted into the document.  Clone the Rule instead.
-      RefPtr<DeclarationBlock> declClone = decl->Clone();
+      RefPtr<css::Declaration> declClone = decl->Clone();
 
       rv = aDst->SetInlineStyleDeclaration(declClone, &valStr, false);
       NS_ENSURE_SUCCESS(rv, rv);
diff --git a/dom/xul/nsXULElement.cpp b/dom/xul/nsXULElement.cpp
index 332e7bb8b4..ee3a50cb88 100644
--- a/dom/xul/nsXULElement.cpp
+++ b/dom/xul/nsXULElement.cpp
@@ -38,7 +38,6 @@
 #include "mozilla/EventListenerManager.h"
 #include "mozilla/EventStateManager.h"
 #include "mozilla/EventStates.h"
-#include "mozilla/DeclarationBlockInlines.h"
 #include "nsFocusManager.h"
 #include "nsHTMLStyleSheet.h"
 #include "nsNameSpaceManager.h"
@@ -54,6 +53,7 @@
 #include "nsIScriptSecurityManager.h"
 #include "nsIServiceManager.h"
 #include "mozilla/css/StyleRule.h"
+#include "mozilla/css/Declaration.h"
 #include "nsIURL.h"
 #include "nsViewManager.h"
 #include "nsIWidget.h"
@@ -348,9 +348,9 @@ nsXULElement::Clone(mozilla::dom::NodeInfo *aNodeInfo, nsINode **aResult) const
 
         // Style rules need to be cloned.
         if (originalValue->Type() == nsAttrValue::eCSSDeclaration) {
-            DeclarationBlock* decl = originalValue->GetCSSDeclarationValue();
+            css::Declaration* decl = originalValue->GetCSSDeclarationValue();
             RefPtr<css::Declaration>
-              declClone = new css::Declaration(*decl->AsGecko());
+              declClone = new css::Declaration(*decl);
 
             nsString stringValue;
             originalValue->ToString(stringValue);
@@ -1900,9 +1900,9 @@ nsXULElement::MakeHeavyweight(nsXULPrototypeElement* aPrototype)
 
         // Style rules need to be cloned.
         if (protoattr->mValue.Type() == nsAttrValue::eCSSDeclaration) {
-            DeclarationBlock* decl = protoattr->mValue.GetCSSDeclarationValue();
+            css::Declaration* decl = protoattr->mValue.GetCSSDeclarationValue();
             RefPtr<css::Declaration>
-              declClone = new css::Declaration(*decl->AsGecko());
+              declClone = new css::Declaration(*decl);
 
             nsString stringValue;
             protoattr->mValue.ToString(stringValue);
diff --git a/editor/libeditor/CSSEditUtils.cpp b/editor/libeditor/CSSEditUtils.cpp
index 976918b7c8..f2aaec3e23 100644
--- a/editor/libeditor/CSSEditUtils.cpp
+++ b/editor/libeditor/CSSEditUtils.cpp
@@ -9,8 +9,8 @@
 #include "mozilla/ChangeStyleTransaction.h"
 #include "mozilla/HTMLEditor.h"
 #include "mozilla/Preferences.h"
-#include "mozilla/DeclarationBlockInlines.h"
 #include "mozilla/css/StyleRule.h"
+#include "mozilla/css/Declaration.h"
 #include "mozilla/dom/Element.h"
 #include "mozilla/mozalloc.h"
 #include "nsAString.h"
@@ -548,7 +548,7 @@ CSSEditUtils::GetCSSInlinePropertyBase(nsINode* aNode,
   }
 
   MOZ_ASSERT(aStyleType == eSpecified);
-  RefPtr<DeclarationBlock> decl = element->GetInlineStyleDeclaration();
+  RefPtr<css::Declaration> decl = element->GetInlineStyleDeclaration();
   if (!decl) {
     return NS_OK;
   }
@@ -556,7 +556,7 @@ CSSEditUtils::GetCSSInlinePropertyBase(nsINode* aNode,
     nsCSSProps::LookupProperty(nsDependentAtomString(aProperty),
                                CSSEnabledState::eForAllContent);
   MOZ_ASSERT(prop != eCSSProperty_UNKNOWN);
-  decl->AsGecko()->GetPropertyValueByID(prop, aValue);
+  decl->GetPropertyValueByID(prop, aValue);
 
   return NS_OK;
 }
diff --git a/layout/style/Declaration.cpp b/layout/style/Declaration.cpp
index 6ef53f5db1..ab2def15fa 100644
--- a/layout/style/Declaration.cpp
+++ b/layout/style/Declaration.cpp
@@ -59,9 +59,14 @@ ImportantStyleData::List(FILE* out, int32_t aIndent) const
 }
 #endif
 
+Declaration::Declaration()
+  : mImmutable(false)
+{
+  mContainer.mRaw = 0;
+}
+
 Declaration::Declaration(const Declaration& aCopy)
-  : DeclarationBlock(aCopy),
-    mOrder(aCopy.mOrder),
+  : mOrder(aCopy.mOrder),
     mVariableOrder(aCopy.mVariableOrder),
     mData(aCopy.mData ? aCopy.mData->Clone() : nullptr),
     mImportantData(aCopy.mImportantData ?
@@ -71,7 +76,8 @@ Declaration::Declaration(const Declaration& aCopy)
         nullptr),
     mImportantVariables(aCopy.mImportantVariables ?
         new CSSVariableDeclarations(*aCopy.mImportantVariables) :
-        nullptr)
+        nullptr),
+    mImmutable(false)
 {
 }
 
@@ -1827,6 +1833,24 @@ Declaration::InitializeEmpty()
   mData = nsCSSCompressedDataBlock::CreateEmptyBlock();
 }
 
+already_AddRefed<Declaration>
+Declaration::Clone() const
+{
+  RefPtr<Declaration> result;
+  result = new Declaration(*this);
+  return result.forget();
+}
+
+already_AddRefed<Declaration>
+Declaration::EnsureMutable()
+{
+  AssertNotExpanded();
+  if (!IsMutable()) {
+    return Clone();
+  }
+  return do_AddRef(this);
+}
+
 size_t
 Declaration::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
 {
diff --git a/layout/style/Declaration.h b/layout/style/Declaration.h
index e3ab95560c..e7701e7380 100644
--- a/layout/style/Declaration.h
+++ b/layout/style/Declaration.h
@@ -18,7 +18,6 @@
 #endif
 
 #include "mozilla/Attributes.h"
-#include "mozilla/DeclarationBlock.h"
 #include "mozilla/MemoryReporting.h"
 #include "CSSVariableDeclarations.h"
 #include "nsCSSDataBlock.h"
@@ -83,8 +82,7 @@ private:
 // be copied before it can be modified, which is taken care of by
 // |EnsureMutable|.
 
-class Declaration final : public DeclarationBlock
-                        , public nsIStyleRule
+class Declaration final : public nsIStyleRule
 {
 public:
   /**
@@ -92,7 +90,7 @@ public:
    * |mData|) and cannot be used until its |CompressFrom| method or
    * |InitializeEmpty| method is called.
    */
-  Declaration() : DeclarationBlock() {}
+  Declaration();
 
   Declaration(const Declaration& aCopy);
 
@@ -104,6 +102,63 @@ private:
   ~Declaration();
 
 public:
+  /**
+   * Return whether |this| may be modified.
+   */
+  bool IsMutable() const {
+    return !mImmutable;
+  }
+
+  /**
+   * Crash if |this| cannot be modified.
+   */
+  void AssertMutable() const {
+    MOZ_ASSERT(IsMutable(), "someone forgot to call EnsureMutable");
+  }
+
+  /**
+   * Mark this declaration as unmodifiable.  It's 'const' so it can
+   * be called from ToString.
+   */
+  void SetImmutable() const { mImmutable = true; }
+
+  /**
+   * Copy |this|, if necessary to ensure that it can be modified.
+   */
+  already_AddRefed<Declaration> EnsureMutable();
+
+  void SetOwningRule(css::Rule* aRule) {
+    MOZ_ASSERT(!mContainer.mOwningRule || !aRule,
+               "should never overwrite one rule with another");
+    mContainer.mOwningRule = aRule;
+  }
+
+  css::Rule* GetOwningRule() const {
+    if (mContainer.mRaw & 0x1) {
+      return nullptr;
+    }
+    return mContainer.mOwningRule;
+  }
+
+  void SetHTMLCSSStyleSheet(nsHTMLCSSStyleSheet* aHTMLCSSStyleSheet) {
+    MOZ_ASSERT(!mContainer.mHTMLCSSStyleSheet || !aHTMLCSSStyleSheet,
+               "should never overwrite one sheet with another");
+    mContainer.mHTMLCSSStyleSheet = aHTMLCSSStyleSheet;
+    if (aHTMLCSSStyleSheet) {
+      mContainer.mRaw |= uintptr_t(1);
+    }
+  }
+
+  nsHTMLCSSStyleSheet* GetHTMLCSSStyleSheet() const {
+    if (!(mContainer.mRaw & 0x1)) {
+      return nullptr;
+    }
+    auto c = mContainer;
+    c.mRaw &= ~uintptr_t(1);
+    return c.mHTMLCSSStyleSheet;
+  }
+
+  already_AddRefed<Declaration> Clone() const;
 
   // nsIStyleRule implementation
   virtual void MapRuleInfoInto(nsRuleData *aRuleData) override;
@@ -367,6 +422,27 @@ public:
   size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
 
 private:
+  union {
+    // We only ever have one of these since we have an
+    // nsHTMLCSSStyleSheet only for style attributes, and style
+    // attributes never have an owning rule.
+
+    // It's an nsHTMLCSSStyleSheet if the low bit is set.
+
+    uintptr_t mRaw;
+
+    // The style rule that owns this declaration.  May be null.
+    css::Rule* mOwningRule;
+
+    // The nsHTMLCSSStyleSheet that is responsible for this declaration.
+    // Only non-null for style attributes.
+    nsHTMLCSSStyleSheet* mHTMLCSSStyleSheet;
+  } mContainer;
+
+  // set when declaration put in the rule tree;
+  // also by ToString (hence the 'mutable').
+  mutable bool mImmutable;
+
   // The order of properties in this declaration.  Longhand properties are
   // represented by their nsCSSPropertyID value, and each custom property (--*)
   // is represented by a value that begins at eCSSProperty_COUNT.
diff --git a/layout/style/DeclarationBlock.h b/layout/style/DeclarationBlock.h
deleted file mode 100644
index 272cdccee3..0000000000
--- a/layout/style/DeclarationBlock.h
+++ /dev/null
@@ -1,140 +0,0 @@
-/* -*- Mode: C++; tab-width: 8; 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/. */
-
-/*
- * representation of a declaration block in a CSS stylesheet, or of
- * a style attribute
- */
-
-#ifndef mozilla_DeclarationBlock_h
-#define mozilla_DeclarationBlock_h
-
-#include "mozilla/DeprecatedUtils.h"
-
-#include "nsCSSPropertyID.h"
-
-class nsHTMLCSSStyleSheet;
-
-namespace mozilla {
-
-namespace css {
-class Declaration;
-class Rule;
-} // namespace css
-
-class DeclarationBlock
-{
-protected:
-  explicit DeclarationBlock()
-    : mImmutable(false) { mContainer.mRaw = 0; }
-
-  DeclarationBlock(const DeclarationBlock& aCopy)
-    : DeclarationBlock() {}
-
-public:
-  MOZ_DECL_DEPRECATED_METHODS(css::Declaration)
-
-  inline MozExternalRefCountType AddRef();
-  inline MozExternalRefCountType Release();
-
-  inline already_AddRefed<DeclarationBlock> Clone() const;
-
-  /**
-   * Return whether |this| may be modified.
-   */
-  bool IsMutable() const {
-    return !mImmutable;
-  }
-
-  /**
-   * Crash if |this| cannot be modified.
-   */
-  void AssertMutable() const {
-    MOZ_ASSERT(IsMutable(), "someone forgot to call EnsureMutable");
-  }
-
-  /**
-   * Mark this declaration as unmodifiable.  It's 'const' so it can
-   * be called from ToString.
-   */
-  void SetImmutable() const { mImmutable = true; }
-
-  /**
-   * Copy |this|, if necessary to ensure that it can be modified.
-   */
-  inline already_AddRefed<DeclarationBlock> EnsureMutable();
-
-  void SetOwningRule(css::Rule* aRule) {
-    MOZ_ASSERT(!mContainer.mOwningRule || !aRule,
-               "should never overwrite one rule with another");
-    mContainer.mOwningRule = aRule;
-  }
-
-  css::Rule* GetOwningRule() const {
-    if (mContainer.mRaw & 0x1) {
-      return nullptr;
-    }
-    return mContainer.mOwningRule;
-  }
-
-  void SetHTMLCSSStyleSheet(nsHTMLCSSStyleSheet* aHTMLCSSStyleSheet) {
-    MOZ_ASSERT(!mContainer.mHTMLCSSStyleSheet || !aHTMLCSSStyleSheet,
-               "should never overwrite one sheet with another");
-    mContainer.mHTMLCSSStyleSheet = aHTMLCSSStyleSheet;
-    if (aHTMLCSSStyleSheet) {
-      mContainer.mRaw |= uintptr_t(1);
-    }
-  }
-
-  nsHTMLCSSStyleSheet* GetHTMLCSSStyleSheet() const {
-    if (!(mContainer.mRaw & 0x1)) {
-      return nullptr;
-    }
-    auto c = mContainer;
-    c.mRaw &= ~uintptr_t(1);
-    return c.mHTMLCSSStyleSheet;
-  }
-
-  inline void ToString(nsAString& aString) const;
-
-  inline uint32_t Count() const;
-  inline bool GetNthProperty(uint32_t aIndex, nsAString& aReturn) const;
-
-  inline void GetPropertyValue(const nsAString& aProperty,
-                               nsAString& aValue) const;
-  inline void GetPropertyValueByID(nsCSSPropertyID aPropID,
-                                   nsAString& aValue) const;
-  inline void GetAuthoredPropertyValue(const nsAString& aProperty,
-                                       nsAString& aValue) const;
-  inline bool GetPropertyIsImportant(const nsAString& aProperty) const;
-  inline void RemoveProperty(const nsAString& aProperty);
-  inline void RemovePropertyByID(nsCSSPropertyID aProperty);
-
-private:
-  union {
-    // We only ever have one of these since we have an
-    // nsHTMLCSSStyleSheet only for style attributes, and style
-    // attributes never have an owning rule.
-
-    // It's an nsHTMLCSSStyleSheet if the low bit is set.
-
-    uintptr_t mRaw;
-
-    // The style rule that owns this declaration.  May be null.
-    css::Rule* mOwningRule;
-
-    // The nsHTMLCSSStyleSheet that is responsible for this declaration.
-    // Only non-null for style attributes.
-    nsHTMLCSSStyleSheet* mHTMLCSSStyleSheet;
-  } mContainer;
-
-  // set when declaration put in the rule tree;
-  // also by ToString (hence the 'mutable').
-  mutable bool mImmutable;
-};
-
-} // namespace mozilla
-
-#endif // mozilla_DeclarationBlock_h
diff --git a/layout/style/DeclarationBlockInlines.h b/layout/style/DeclarationBlockInlines.h
deleted file mode 100644
index 17f7ddb5e0..0000000000
--- a/layout/style/DeclarationBlockInlines.h
+++ /dev/null
@@ -1,106 +0,0 @@
-/* -*- Mode: C++; tab-width: 8; 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/. */
-
-#ifndef mozilla_DeclarationBlockInlines_h
-#define mozilla_DeclarationBlockInlines_h
-
-#include "mozilla/css/Declaration.h"
-
-namespace mozilla {
-
-MOZ_DEFINE_DEPRECATED_METHODS(DeclarationBlock, css::Declaration)
-
-MozExternalRefCountType
-DeclarationBlock::AddRef()
-{
-  return AsGecko()->AddRef();
-}
-
-MozExternalRefCountType
-DeclarationBlock::Release()
-{
-  return AsGecko()->Release();
-}
-
-already_AddRefed<DeclarationBlock>
-DeclarationBlock::Clone() const
-{
-  RefPtr<DeclarationBlock> result;
-  result = new css::Declaration(*AsGecko());
-  return result.forget();
-}
-
-already_AddRefed<DeclarationBlock>
-DeclarationBlock::EnsureMutable()
-{
-#ifdef DEBUG
-  AsGecko()->AssertNotExpanded();
-#endif
-  if (!IsMutable()) {
-    return Clone();
-  }
-  return do_AddRef(this);
-}
-
-void
-DeclarationBlock::ToString(nsAString& aString) const
-{
-  AsGecko()->ToString(aString);
-}
-
-uint32_t
-DeclarationBlock::Count() const
-{
-  return AsGecko()->Count();
-}
-
-bool
-DeclarationBlock::GetNthProperty(uint32_t aIndex, nsAString& aReturn) const
-{
-  return AsGecko()->GetNthProperty(aIndex, aReturn);
-}
-
-void
-DeclarationBlock::GetPropertyValue(const nsAString& aProperty,
-                                   nsAString& aValue) const
-{
-  AsGecko()->GetPropertyValue(aProperty, aValue);
-}
-
-void
-DeclarationBlock::GetPropertyValueByID(nsCSSPropertyID aPropID,
-                                       nsAString& aValue) const
-{
-  AsGecko()->GetPropertyValueByID(aPropID, aValue);
-}
-
-void
-DeclarationBlock::GetAuthoredPropertyValue(const nsAString& aProperty,
-                                           nsAString& aValue) const
-{
-  AsGecko()->GetAuthoredPropertyValue(aProperty, aValue);
-}
-
-bool
-DeclarationBlock::GetPropertyIsImportant(const nsAString& aProperty) const
-{
-  return AsGecko()->GetPropertyIsImportant(aProperty);
-}
-
-void
-DeclarationBlock::RemoveProperty(const nsAString& aProperty)
-{
-  AsGecko()->RemoveProperty(aProperty);
-}
-
-void
-DeclarationBlock::RemovePropertyByID(nsCSSPropertyID aProperty)
-{
-  AsGecko()->RemovePropertyByID(aProperty);
-}
-
-} // namespace mozilla
-
-#endif // mozilla_DeclarationBlockInlines_h
diff --git a/layout/style/StyleRule.cpp b/layout/style/StyleRule.cpp
index f7a267776d..5c3ec354d5 100644
--- a/layout/style/StyleRule.cpp
+++ b/layout/style/StyleRule.cpp
@@ -11,7 +11,6 @@
 
 #include "mozilla/css/StyleRule.h"
 
-#include "mozilla/DeclarationBlockInlines.h"
 #include "mozilla/StyleSheetInlines.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/css/GroupRule.h"
@@ -1113,8 +1112,8 @@ public:
 
   NS_IMETHOD GetParentRule(nsIDOMCSSRule **aParent) override;
   void DropReference(void);
-  virtual DeclarationBlock* GetCSSDeclaration(Operation aOperation) override;
-  virtual nsresult SetCSSDeclaration(DeclarationBlock* aDecl) override;
+  virtual css::Declaration* GetCSSDeclaration(Operation aOperation) override;
+  virtual nsresult SetCSSDeclaration(css::Declaration* aDecl) override;
   virtual void GetCSSParsingEnvironment(CSSParsingEnvironment& aCSSParseEnv) override;
   virtual nsIDocument* DocToUpdate() override;
 
@@ -1227,7 +1226,7 @@ DOMCSSDeclarationImpl::DropReference(void)
   mRule = nullptr;
 }
 
-DeclarationBlock*
+css::Declaration*
 DOMCSSDeclarationImpl::GetCSSDeclaration(Operation aOperation)
 {
   if (mRule) {
@@ -1264,7 +1263,7 @@ DOMCSSDeclarationImpl::GetParentRule(nsIDOMCSSRule **aParent)
 }
 
 nsresult
-DOMCSSDeclarationImpl::SetCSSDeclaration(DeclarationBlock* aDecl)
+DOMCSSDeclarationImpl::SetCSSDeclaration(css::Declaration* aDecl)
 {
   NS_PRECONDITION(mRule,
          "can only be called when |GetCSSDeclaration| returned a declaration");
@@ -1277,7 +1276,7 @@ DOMCSSDeclarationImpl::SetCSSDeclaration(DeclarationBlock* aDecl)
 
   mozAutoDocUpdate updateBatch(doc, UPDATE_STYLE, true);
 
-  mRule->SetDeclaration(aDecl->AsGecko());
+  mRule->SetDeclaration(aDecl);
 
   if (sheet) {
     sheet->DidDirty();
diff --git a/layout/style/moz.build b/layout/style/moz.build
index 7595212966..9452ff15ce 100644
--- a/layout/style/moz.build
+++ b/layout/style/moz.build
@@ -79,8 +79,6 @@ EXPORTS.mozilla += [
     'CSSVariableDeclarations.h',
     'CSSVariableResolver.h',
     'CSSVariableValues.h',
-    'DeclarationBlock.h',
-    'DeclarationBlockInlines.h',
     'DeprecatedUtils.h',
     'HandleRefPtr.h',
     'IncrementalClearCOMRuleArray.h',
diff --git a/layout/style/nsCSSRules.cpp b/layout/style/nsCSSRules.cpp
index dc79e471d8..1642516979 100644
--- a/layout/style/nsCSSRules.cpp
+++ b/layout/style/nsCSSRules.cpp
@@ -29,7 +29,7 @@
 #include "nsContentUtils.h"
 #include "nsError.h"
 #include "nsStyleUtil.h"
-#include "mozilla/DeclarationBlockInlines.h"
+#include "mozilla/css/Declaration.h"
 #include "nsCSSParser.h"
 #include "nsDOMClassInfoID.h"
 #include "mozilla/dom/CSSStyleDeclarationBinding.h"
@@ -1953,7 +1953,7 @@ NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsCSSKeyframeStyleDeclaration)
   NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
 NS_INTERFACE_MAP_END_INHERITING(nsDOMCSSDeclaration)
 
-DeclarationBlock*
+css::Declaration*
 nsCSSKeyframeStyleDeclaration::GetCSSDeclaration(Operation aOperation)
 {
   if (mRule) {
@@ -1979,10 +1979,10 @@ nsCSSKeyframeStyleDeclaration::GetParentRule(nsIDOMCSSRule **aParent)
 }
 
 nsresult
-nsCSSKeyframeStyleDeclaration::SetCSSDeclaration(DeclarationBlock* aDecl)
+nsCSSKeyframeStyleDeclaration::SetCSSDeclaration(css::Declaration* aDecl)
 {
   MOZ_ASSERT(aDecl, "must be non-null");
-  mRule->ChangeDeclaration(aDecl->AsGecko());
+  mRule->ChangeDeclaration(aDecl);
   return NS_OK;
 }
 
@@ -2511,7 +2511,7 @@ NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsCSSPageStyleDeclaration)
   NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
 NS_INTERFACE_MAP_END_INHERITING(nsDOMCSSDeclaration)
 
-DeclarationBlock*
+css::Declaration*
 nsCSSPageStyleDeclaration::GetCSSDeclaration(Operation aOperation)
 {
   if (mRule) {
@@ -2537,10 +2537,10 @@ nsCSSPageStyleDeclaration::GetParentRule(nsIDOMCSSRule** aParent)
 }
 
 nsresult
-nsCSSPageStyleDeclaration::SetCSSDeclaration(DeclarationBlock* aDecl)
+nsCSSPageStyleDeclaration::SetCSSDeclaration(css::Declaration* aDecl)
 {
   MOZ_ASSERT(aDecl, "must be non-null");
-  mRule->ChangeDeclaration(aDecl->AsGecko());
+  mRule->ChangeDeclaration(aDecl);
   return NS_OK;
 }
 
diff --git a/layout/style/nsCSSRules.h b/layout/style/nsCSSRules.h
index 3e4eef039c..9c3196575c 100644
--- a/layout/style/nsCSSRules.h
+++ b/layout/style/nsCSSRules.h
@@ -360,8 +360,8 @@ public:
 
   NS_IMETHOD GetParentRule(nsIDOMCSSRule **aParent) override;
   void DropReference() { mRule = nullptr; }
-  virtual mozilla::DeclarationBlock* GetCSSDeclaration(Operation aOperation) override;
-  virtual nsresult SetCSSDeclaration(mozilla::DeclarationBlock* aDecl) override;
+  virtual mozilla::css::Declaration* GetCSSDeclaration(Operation aOperation) override;
+  virtual nsresult SetCSSDeclaration(mozilla::css::Declaration* aDecl) override;
   virtual void GetCSSParsingEnvironment(CSSParsingEnvironment& aCSSParseEnv) override;
   virtual nsIDocument* DocToUpdate() override;
 
@@ -491,8 +491,8 @@ public:
 
   NS_IMETHOD GetParentRule(nsIDOMCSSRule **aParent) override;
   void DropReference() { mRule = nullptr; }
-  virtual mozilla::DeclarationBlock* GetCSSDeclaration(Operation aOperation) override;
-  virtual nsresult SetCSSDeclaration(mozilla::DeclarationBlock* aDecl) override;
+  virtual mozilla::css::Declaration* GetCSSDeclaration(Operation aOperation) override;
+  virtual nsresult SetCSSDeclaration(mozilla::css::Declaration* aDecl) override;
   virtual void GetCSSParsingEnvironment(CSSParsingEnvironment& aCSSParseEnv) override;
   virtual nsIDocument* DocToUpdate() override;
 
diff --git a/layout/style/nsComputedDOMStyle.cpp b/layout/style/nsComputedDOMStyle.cpp
index 191bffcfb5..006d8ffbba 100644
--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -578,7 +578,7 @@ nsComputedDOMStyle::GetPresShellForContent(nsIContent* aContent)
 // nsDOMCSSDeclaration abstract methods which should never be called
 // on a nsComputedDOMStyle object, but must be defined to avoid
 // compile errors.
-DeclarationBlock*
+mozilla::css::Declaration*
 nsComputedDOMStyle::GetCSSDeclaration(Operation)
 {
   NS_RUNTIMEABORT("called nsComputedDOMStyle::GetCSSDeclaration");
@@ -586,7 +586,7 @@ nsComputedDOMStyle::GetCSSDeclaration(Operation)
 }
 
 nsresult
-nsComputedDOMStyle::SetCSSDeclaration(DeclarationBlock*)
+nsComputedDOMStyle::SetCSSDeclaration(mozilla::css::Declaration*)
 {
   NS_RUNTIMEABORT("called nsComputedDOMStyle::SetCSSDeclaration");
   return NS_ERROR_FAILURE;
diff --git a/layout/style/nsComputedDOMStyle.h b/layout/style/nsComputedDOMStyle.h
index 363de2a8fe..996f8ffc42 100644
--- a/layout/style/nsComputedDOMStyle.h
+++ b/layout/style/nsComputedDOMStyle.h
@@ -113,8 +113,8 @@ public:
   // nsDOMCSSDeclaration abstract methods which should never be called
   // on a nsComputedDOMStyle object, but must be defined to avoid
   // compile errors.
-  virtual mozilla::DeclarationBlock* GetCSSDeclaration(Operation) override;
-  virtual nsresult SetCSSDeclaration(mozilla::DeclarationBlock*) override;
+  virtual mozilla::css::Declaration* GetCSSDeclaration(Operation) override;
+  virtual nsresult SetCSSDeclaration(mozilla::css::Declaration*) override;
   virtual nsIDocument* DocToUpdate() override;
   virtual void GetCSSParsingEnvironment(CSSParsingEnvironment& aCSSParseEnv) override;
 
diff --git a/layout/style/nsDOMCSSAttrDeclaration.cpp b/layout/style/nsDOMCSSAttrDeclaration.cpp
index f2117dc0df..7b68990333 100644
--- a/layout/style/nsDOMCSSAttrDeclaration.cpp
+++ b/layout/style/nsDOMCSSAttrDeclaration.cpp
@@ -19,7 +19,6 @@
 #include "nsIFrame.h"
 #include "ActiveLayerTracker.h"
 #include "StyleSetHandle.h"
-#include "DeclarationBlockInlines.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
@@ -73,7 +72,7 @@ NS_IMPL_CYCLE_COLLECTING_ADDREF(nsDOMCSSAttributeDeclaration)
 NS_IMPL_CYCLE_COLLECTING_RELEASE(nsDOMCSSAttributeDeclaration)
 
 nsresult
-nsDOMCSSAttributeDeclaration::SetCSSDeclaration(DeclarationBlock* aDecl)
+nsDOMCSSAttributeDeclaration::SetCSSDeclaration(css::Declaration* aDecl)
 {
   NS_ASSERTION(mElement, "Must have Element to set the declaration!");
   return mIsSMILOverride
@@ -89,13 +88,13 @@ nsDOMCSSAttributeDeclaration::DocToUpdate()
   return mElement->OwnerDoc();
 }
 
-DeclarationBlock*
+css::Declaration*
 nsDOMCSSAttributeDeclaration::GetCSSDeclaration(Operation aOperation)
 {
   if (!mElement)
     return nullptr;
 
-  DeclarationBlock* declaration;
+  css::Declaration* declaration;
   if (mIsSMILOverride) {
     declaration = mElement->GetSMILOverrideStyleDeclaration();
   } else {
@@ -132,9 +131,9 @@ nsDOMCSSAttributeDeclaration::GetCSSDeclaration(Operation aOperation)
   }
 
   // cannot fail
-  RefPtr<DeclarationBlock> decl;
+  RefPtr<css::Declaration> decl;
   decl = new css::Declaration();
-  decl->AsGecko()->InitializeEmpty();
+  decl->InitializeEmpty();
 
   // this *can* fail (inside SetAttrAndNotify, at least).
   nsresult rv;
diff --git a/layout/style/nsDOMCSSAttrDeclaration.h b/layout/style/nsDOMCSSAttrDeclaration.h
index 482d665ea7..e10f99fc4c 100644
--- a/layout/style/nsDOMCSSAttrDeclaration.h
+++ b/layout/style/nsDOMCSSAttrDeclaration.h
@@ -30,7 +30,7 @@ public:
 
   // If GetCSSDeclaration returns non-null, then the decl it returns
   // is owned by our current style rule.
-  virtual mozilla::DeclarationBlock* GetCSSDeclaration(Operation aOperation) override;
+  virtual mozilla::css::Declaration* GetCSSDeclaration(Operation aOperation) override;
   virtual void GetCSSParsingEnvironment(CSSParsingEnvironment& aCSSParseEnv) override;
   NS_IMETHOD GetParentRule(nsIDOMCSSRule **aParent) override;
 
@@ -44,7 +44,7 @@ public:
 protected:
   ~nsDOMCSSAttributeDeclaration();
 
-  virtual nsresult SetCSSDeclaration(mozilla::DeclarationBlock* aDecl) override;
+  virtual nsresult SetCSSDeclaration(mozilla::css::Declaration* aDecl) override;
   virtual nsIDocument* DocToUpdate() override;
 
   RefPtr<Element> mElement;
diff --git a/layout/style/nsDOMCSSDeclaration.cpp b/layout/style/nsDOMCSSDeclaration.cpp
index 26faa16d4c..25af0275b7 100644
--- a/layout/style/nsDOMCSSDeclaration.cpp
+++ b/layout/style/nsDOMCSSDeclaration.cpp
@@ -8,10 +8,9 @@
 #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/css/Declaration.h"
 #include "mozilla/dom/CSS2PropertiesBinding.h"
 #include "nsCSSProps.h"
 #include "nsCOMPtr.h"
@@ -49,7 +48,7 @@ nsDOMCSSDeclaration::GetPropertyValue(const nsCSSPropertyID aPropID,
                   "Should never pass eCSSProperty_UNKNOWN around");
 
   aValue.Truncate();
-  if (DeclarationBlock* decl = GetCSSDeclaration(eOperation_Read)) {
+  if (css::Declaration* decl = GetCSSDeclaration(eOperation_Read)) {
     decl->GetPropertyValueByID(aPropID, aValue);
   }
   return NS_OK;
@@ -97,7 +96,7 @@ nsDOMCSSDeclaration::SetPropertyValue(const nsCSSPropertyID aPropID,
 NS_IMETHODIMP
 nsDOMCSSDeclaration::GetCssText(nsAString& aCssText)
 {
-  DeclarationBlock* decl = GetCSSDeclaration(eOperation_Read);
+  css::Declaration* decl = GetCSSDeclaration(eOperation_Read);
   aCssText.Truncate();
 
   if (decl) {
@@ -112,7 +111,7 @@ 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.
-  DeclarationBlock* olddecl = GetCSSDeclaration(eOperation_Modify);
+  css::Declaration* olddecl = GetCSSDeclaration(eOperation_Modify);
   if (!olddecl) {
     return NS_ERROR_NOT_AVAILABLE;
   }
@@ -130,7 +129,7 @@ nsDOMCSSDeclaration::SetCssText(const nsAString& aCssText)
   // rule (see stack in bug 209575).
   mozAutoDocConditionalContentUpdateBatch autoUpdate(DocToUpdate(), true);
 
-  RefPtr<DeclarationBlock> newdecl;
+  RefPtr<css::Declaration> newdecl;
   RefPtr<css::Declaration> decl(new css::Declaration());
   decl->InitializeEmpty();
   nsCSSParser cssParser(env.mCSSLoader);
@@ -149,7 +148,7 @@ nsDOMCSSDeclaration::SetCssText(const nsAString& aCssText)
 NS_IMETHODIMP
 nsDOMCSSDeclaration::GetLength(uint32_t* aLength)
 {
-  DeclarationBlock* decl = GetCSSDeclaration(eOperation_Read);
+  css::Declaration* decl = GetCSSDeclaration(eOperation_Read);
 
   if (decl) {
     *aLength = decl->Count();
@@ -171,7 +170,7 @@ nsDOMCSSDeclaration::GetPropertyCSSValue(const nsAString& aPropertyName, ErrorRe
 void
 nsDOMCSSDeclaration::IndexedGetter(uint32_t aIndex, bool& aFound, nsAString& aPropName)
 {
-  DeclarationBlock* decl = GetCSSDeclaration(eOperation_Read);
+  css::Declaration* decl = GetCSSDeclaration(eOperation_Read);
   aFound = decl && decl->GetNthProperty(aIndex, aPropName);
 }
 
@@ -180,7 +179,7 @@ nsDOMCSSDeclaration::GetPropertyValue(const nsAString& aPropertyName,
                                       nsAString& aReturn)
 {
   aReturn.Truncate();
-  if (DeclarationBlock* decl = GetCSSDeclaration(eOperation_Read)) {
+  if (css::Declaration* decl = GetCSSDeclaration(eOperation_Read)) {
     decl->GetPropertyValue(aPropertyName, aReturn);
   }
   return NS_OK;
@@ -190,7 +189,7 @@ NS_IMETHODIMP
 nsDOMCSSDeclaration::GetAuthoredPropertyValue(const nsAString& aPropertyName,
                                               nsAString& aReturn)
 {
-  if (DeclarationBlock* decl = GetCSSDeclaration(eOperation_Read)) {
+  if (css::Declaration* decl = GetCSSDeclaration(eOperation_Read)) {
     decl->GetAuthoredPropertyValue(aPropertyName, aReturn);
   }
   return NS_OK;
@@ -200,7 +199,7 @@ NS_IMETHODIMP
 nsDOMCSSDeclaration::GetPropertyPriority(const nsAString& aPropertyName,
                                          nsAString& aReturn)
 {
-  DeclarationBlock* decl = GetCSSDeclaration(eOperation_Read);
+  css::Declaration* decl = GetCSSDeclaration(eOperation_Read);
 
   aReturn.Truncate();
   if (decl && decl->GetPropertyIsImportant(aPropertyName)) {
@@ -276,7 +275,7 @@ nsDOMCSSDeclaration::ParsePropertyValue(const nsCSSPropertyID aPropID,
                                         const nsAString& aPropValue,
                                         bool aIsImportant)
 {
-  DeclarationBlock* olddecl = GetCSSDeclaration(eOperation_Modify);
+  css::Declaration* olddecl = GetCSSDeclaration(eOperation_Modify);
   if (!olddecl) {
     return NS_ERROR_NOT_AVAILABLE;
   }
@@ -293,13 +292,13 @@ nsDOMCSSDeclaration::ParsePropertyValue(const nsCSSPropertyID aPropID,
   // between when we mutate the declaration and when we set the new
   // rule (see stack in bug 209575).
   mozAutoDocConditionalContentUpdateBatch autoUpdate(DocToUpdate(), true);
-  RefPtr<DeclarationBlock> decl = olddecl->EnsureMutable();
+  RefPtr<css::Declaration> decl = olddecl->EnsureMutable();
 
   bool changed;
   nsCSSParser cssParser(env.mCSSLoader);
   cssParser.ParseProperty(aPropID, aPropValue,
                           env.mSheetURI, env.mBaseURI, env.mPrincipal,
-                          decl->AsGecko(), &changed, aIsImportant);
+                          decl, &changed, aIsImportant);
   if (!changed) {
     // Parsing failed -- but we don't throw an exception for that.
     return NS_OK;
@@ -315,7 +314,7 @@ nsDOMCSSDeclaration::ParseCustomPropertyValue(const nsAString& aPropertyName,
 {
   MOZ_ASSERT(nsCSSProps::IsCustomPropertyName(aPropertyName));
 
-  DeclarationBlock* olddecl = GetCSSDeclaration(eOperation_Modify);
+  css::Declaration* olddecl = GetCSSDeclaration(eOperation_Modify);
   if (!olddecl) {
     return NS_ERROR_NOT_AVAILABLE;
   }
@@ -332,13 +331,13 @@ nsDOMCSSDeclaration::ParseCustomPropertyValue(const nsAString& aPropertyName,
   // between when we mutate the declaration and when we set the new
   // rule (see stack in bug 209575).
   mozAutoDocConditionalContentUpdateBatch autoUpdate(DocToUpdate(), true);
-  RefPtr<DeclarationBlock> decl = olddecl->EnsureMutable();
+  RefPtr<css::Declaration> decl = olddecl->EnsureMutable();
 
   bool changed;
   auto propName = Substring(aPropertyName, CSS_CUSTOM_NAME_PREFIX_LENGTH);
   nsCSSParser cssParser(env.mCSSLoader);
   cssParser.ParseVariable(propName, aPropValue, env.mSheetURI,
-                          env.mBaseURI, env.mPrincipal, decl->AsGecko(),
+                          env.mBaseURI, env.mPrincipal, decl,
                           &changed, aIsImportant);
   if (!changed) {
     // Parsing failed -- but we don't throw an exception for that.
@@ -351,7 +350,7 @@ nsDOMCSSDeclaration::ParseCustomPropertyValue(const nsAString& aPropertyName,
 nsresult
 nsDOMCSSDeclaration::RemovePropertyInternal(nsCSSPropertyID aPropID)
 {
-  DeclarationBlock* olddecl = GetCSSDeclaration(eOperation_RemoveProperty);
+  css::Declaration* olddecl = GetCSSDeclaration(eOperation_RemoveProperty);
   if (!olddecl) {
     return NS_OK; // no decl, so nothing to remove
   }
@@ -363,7 +362,7 @@ nsDOMCSSDeclaration::RemovePropertyInternal(nsCSSPropertyID aPropID)
   // rule (see stack in bug 209575).
   mozAutoDocConditionalContentUpdateBatch autoUpdate(DocToUpdate(), true);
 
-  RefPtr<DeclarationBlock> decl = olddecl->EnsureMutable();
+  RefPtr<css::Declaration> decl = olddecl->EnsureMutable();
   decl->RemovePropertyByID(aPropID);
   return SetCSSDeclaration(decl);
 }
@@ -371,7 +370,7 @@ nsDOMCSSDeclaration::RemovePropertyInternal(nsCSSPropertyID aPropID)
 nsresult
 nsDOMCSSDeclaration::RemovePropertyInternal(const nsAString& aPropertyName)
 {
-  DeclarationBlock* olddecl = GetCSSDeclaration(eOperation_RemoveProperty);
+  css::Declaration* olddecl = GetCSSDeclaration(eOperation_RemoveProperty);
   if (!olddecl) {
     return NS_OK; // no decl, so nothing to remove
   }
@@ -383,7 +382,7 @@ nsDOMCSSDeclaration::RemovePropertyInternal(const nsAString& aPropertyName)
   // rule (see stack in bug 209575).
   mozAutoDocConditionalContentUpdateBatch autoUpdate(DocToUpdate(), true);
 
-  RefPtr<DeclarationBlock> decl = olddecl->EnsureMutable();
+  RefPtr<css::Declaration> decl = olddecl->EnsureMutable();
   decl->RemoveProperty(aPropertyName);
   return SetCSSDeclaration(decl);
 }
diff --git a/layout/style/nsDOMCSSDeclaration.h b/layout/style/nsDOMCSSDeclaration.h
index 5b2308e4af..c0b296709d 100644
--- a/layout/style/nsDOMCSSDeclaration.h
+++ b/layout/style/nsDOMCSSDeclaration.h
@@ -20,10 +20,10 @@ struct JSContext;
 class JSObject;
 
 namespace mozilla {
-class DeclarationBlock;
 namespace css {
 class Loader;
 class Rule;
+class Declaration;
 } // namespace css
 } // namespace mozilla
 
@@ -119,8 +119,8 @@ protected:
     // AttributeWillChange.
     eOperation_RemoveProperty
   };
-  virtual mozilla::DeclarationBlock* GetCSSDeclaration(Operation aOperation) = 0;
-  virtual nsresult SetCSSDeclaration(mozilla::DeclarationBlock* aDecl) = 0;
+  virtual mozilla::css::Declaration* GetCSSDeclaration(Operation aOperation) = 0;
+  virtual nsresult SetCSSDeclaration(mozilla::css::Declaration* aDecl) = 0;
   // Document that we must call BeginUpdate/EndUpdate on around the
   // calls to SetCSSDeclaration and the style rule mutation that leads
   // to it.
diff --git a/layout/style/nsHTMLCSSStyleSheet.cpp b/layout/style/nsHTMLCSSStyleSheet.cpp
index efe14c120e..4f68e6741b 100644
--- a/layout/style/nsHTMLCSSStyleSheet.cpp
+++ b/layout/style/nsHTMLCSSStyleSheet.cpp
@@ -10,7 +10,7 @@
 #include "nsHTMLCSSStyleSheet.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/css/StyleRule.h"
-#include "mozilla/DeclarationBlockInlines.h"
+#include "mozilla/css/Declaration.h"
 #include "nsIStyleRuleProcessor.h"
 #include "nsPresContext.h"
 #include "nsRuleWalker.h"
@@ -39,7 +39,7 @@ nsHTMLCSSStyleSheet::~nsHTMLCSSStyleSheet()
     // Ideally we'd just call MiscContainer::Evict, but we can't do that since
     // we're iterating the hashtable.
     if (value->mType == nsAttrValue::eCSSDeclaration) {
-      DeclarationBlock* declaration = value->mValue.mCSSDeclaration;
+      css::Declaration* declaration = value->mValue.mCSSDeclaration;
       declaration->SetHTMLCSSStyleSheet(nullptr);
     } else {
       MOZ_ASSERT_UNREACHABLE("unexpected cached nsAttrValue type");
@@ -65,10 +65,10 @@ nsHTMLCSSStyleSheet::ElementRulesMatching(nsPresContext* aPresContext,
                                           nsRuleWalker* aRuleWalker)
 {
   // just get the one and only style rule from the content's STYLE attribute
-  DeclarationBlock* declaration = aElement->GetInlineStyleDeclaration();
+  css::Declaration* declaration = aElement->GetInlineStyleDeclaration();
   if (declaration) {
     declaration->SetImmutable();
-    aRuleWalker->Forward(declaration->AsGecko());
+    aRuleWalker->Forward(declaration);
   }
 
   declaration = aElement->GetSMILOverrideStyleDeclaration();
@@ -78,7 +78,7 @@ nsHTMLCSSStyleSheet::ElementRulesMatching(nsPresContext* aPresContext,
       // Animation restyle (or non-restyle traversal of rules)
       // Now we can walk SMIL overrride style, without triggering transitions.
       declaration->SetImmutable();
-      aRuleWalker->Forward(declaration->AsGecko());
+      aRuleWalker->Forward(declaration);
     }
   }
 }
@@ -94,10 +94,10 @@ nsHTMLCSSStyleSheet::PseudoElementRulesMatching(Element* aPseudoElement,
   MOZ_ASSERT(aPseudoElement);
 
   // just get the one and only style rule from the content's STYLE attribute
-  DeclarationBlock* declaration = aPseudoElement->GetInlineStyleDeclaration();
+  css::Declaration* declaration = aPseudoElement->GetInlineStyleDeclaration();
   if (declaration) {
     declaration->SetImmutable();
-    aRuleWalker->Forward(declaration->AsGecko());
+    aRuleWalker->Forward(declaration);
   }
 }