--- a/dom/base/nsINode.h
+++ b/dom/base/nsINode.h
@@ -277,17 +277,16 @@ private:
 // defined, it is inherited from nsINode.
 // This macro isn't actually specific to nodes, and bug 956400 will move it into MFBT.
 #define NS_DECL_SIZEOF_EXCLUDING_THIS \
   virtual size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const override;
 
 // Categories of node properties
 // 0 is global.
 #define DOM_USER_DATA         1
-#define SMIL_MAPPED_ATTR_ANIMVAL 2
 
 // IID for the nsINode interface
 #define NS_INODE_IID \
 { 0x70ba4547, 0x7699, 0x44fc, \
   { 0xb3, 0x20, 0x52, 0xdb, 0xe3, 0xd1, 0xf9, 0x0a } }
 
 /**
  * An internal interface that abstracts some DOMNode-related parts that both

--- a/dom/smil/moz.build
+++ b/dom/smil/moz.build
@@ -14,17 +14,16 @@ EXPORTS += [
     'nsISMILType.h',
     'nsSMILAnimationController.h',
     'nsSMILAnimationFunction.h',
     'nsSMILCompositorTable.h',
     'nsSMILCSSProperty.h',
     'nsSMILInstanceTime.h',
     'nsSMILInterval.h',
     'nsSMILKeySpline.h',
-    'nsSMILMappedAttribute.h',
     'nsSMILMilestone.h',
     'nsSMILNullType.h',
     'nsSMILRepeatCount.h',
     'nsSMILSetAnimationFunction.h',
     'nsSMILTargetIdentifier.h',
     'nsSMILTimeContainer.h',
     'nsSMILTimedElement.h',
     'nsSMILTimeValue.h',
@@ -43,17 +42,16 @@ UNIFIED_SOURCES += [
     'nsSMILAnimationFunction.cpp',
     'nsSMILCompositor.cpp',
     'nsSMILCSSProperty.cpp',
     'nsSMILCSSValueType.cpp',
     'nsSMILFloatType.cpp',
     'nsSMILInstanceTime.cpp',
     'nsSMILInterval.cpp',
     'nsSMILKeySpline.cpp',
-    'nsSMILMappedAttribute.cpp',
     'nsSMILNullType.cpp',
     'nsSMILParserUtils.cpp',
     'nsSMILRepeatCount.cpp',
     'nsSMILSetAnimationFunction.cpp',
     'nsSMILTimeContainer.cpp',
     'nsSMILTimedElement.cpp',
     'nsSMILTimeValue.cpp',
     'nsSMILTimeValueSpec.cpp',

deleted file mode 100644
--- a/dom/smil/nsSMILMappedAttribute.cpp
+++ /dev/null
@@ -1,150 +0,0 @@
-/* -*- 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/. */
-
-/* representation of a SMIL-animatable mapped attribute on an element */
-#include "nsSMILMappedAttribute.h"
-#include "nsContentUtils.h"
-#include "nsError.h" // For NS_PROPTABLE_PROP_OVERWRITTEN
-#include "nsSMILValue.h"
-#include "nsSMILCSSValueType.h"
-#include "nsIDocument.h"
-#include "nsIPresShell.h"
-#include "nsCSSProps.h"
-#include "mozilla/dom/Element.h"
-
-// Callback function, for freeing string buffers stored in property table
-static void
-ReleaseStringBufferPropertyValue(void*    aObject,       /* unused */
-                                 nsIAtom* aPropertyName, /* unused */
-                                 void*    aPropertyValue,
-                                 void*    aData          /* unused */)
-{
-  nsStringBuffer* buf = static_cast<nsStringBuffer*>(aPropertyValue);
-  buf->Release();
-}
-
-
-nsresult
-nsSMILMappedAttribute::ValueFromString(const nsAString& aStr,
-                                       const mozilla::dom::SVGAnimationElement* aSrcElement,
-                                       nsSMILValue& aValue,
-                                       bool& aPreventCachingOfSandwich) const
-{
-  NS_ENSURE_TRUE(IsPropertyAnimatable(mPropID), NS_ERROR_FAILURE);
-
-  nsSMILCSSValueType::ValueFromString(mPropID, mElement, aStr, aValue,
-                                      &aPreventCachingOfSandwich);
-  return aValue.IsNull() ? NS_ERROR_FAILURE : NS_OK;
-}
-
-nsSMILValue
-nsSMILMappedAttribute::GetBaseValue() const
-{
-  nsAutoString baseStringValue;
-  RefPtr<nsIAtom> attrName = GetAttrNameAtom();
-  bool success = mElement->GetAttr(kNameSpaceID_None, attrName,
-                                     baseStringValue);
-  nsSMILValue baseValue;
-  if (success) {
-    // For base values, we don't need to worry whether the value returned is
-    // context-sensitive or not since the compositor will take care of comparing
-    // the returned (computed) base value and its cached value and determining
-    // if an update is required or not.
-    nsSMILCSSValueType::ValueFromString(mPropID, mElement,
-                                        baseStringValue, baseValue, nullptr);
-  } else {
-    // Attribute is unset -- use computed value.
-    // FIRST: Temporarily clear animated value, to make sure it doesn't pollute
-    // the computed value. (We want base value, _without_ animations applied.)
-    void* buf = mElement->UnsetProperty(SMIL_MAPPED_ATTR_ANIMVAL,
-                                        attrName, nullptr);
-    FlushChangesToTargetAttr();
-
-    // SECOND: we use nsSMILCSSProperty::GetBaseValue to look up the property's
-    // computed value.  NOTE: This call will temporarily clear the SMIL
-    // override-style for the corresponding CSS property on our target element.
-    // This prevents any animations that target the CSS property from affecting
-    // animations that target the mapped attribute.
-    baseValue = nsSMILCSSProperty::GetBaseValue();
-
-    // FINALLY: If we originally had an animated value set, then set it again.
-    if (buf) {
-      mElement->SetProperty(SMIL_MAPPED_ATTR_ANIMVAL, attrName, buf,
-                            ReleaseStringBufferPropertyValue);
-      FlushChangesToTargetAttr();
-    }
-  }
-  return baseValue;
-}
-
-nsresult
-nsSMILMappedAttribute::SetAnimValue(const nsSMILValue& aValue)
-{
-  NS_ENSURE_TRUE(IsPropertyAnimatable(mPropID), NS_ERROR_FAILURE);
-
-  // Convert nsSMILValue to string
-  nsAutoString valStr;
-  if (!nsSMILCSSValueType::ValueToString(aValue, valStr)) {
-    NS_WARNING("Failed to convert nsSMILValue for mapped attr into a string");
-    return NS_ERROR_FAILURE;
-  }
-
-  RefPtr<nsIAtom> attrName = GetAttrNameAtom();
-  nsStringBuffer* oldValStrBuf = static_cast<nsStringBuffer*>
-    (mElement->GetProperty(SMIL_MAPPED_ATTR_ANIMVAL, attrName));
-  if (oldValStrBuf) {
-    nsString oldValStr;
-    nsContentUtils::PopulateStringFromStringBuffer(oldValStrBuf, oldValStr);
-    if (valStr.Equals(oldValStr)) {
-      // New animated value is the same as the old; nothing to do.
-      return NS_OK;
-    }
-  }
-
-  // Set the string as this mapped attribute's animated value.
-  nsStringBuffer* valStrBuf =
-    nsCSSValue::BufferFromString(nsString(valStr)).take();
-  nsresult rv = mElement->SetProperty(SMIL_MAPPED_ATTR_ANIMVAL,
-                                      attrName, valStrBuf,
-                                      ReleaseStringBufferPropertyValue);
-  if (rv == NS_PROPTABLE_PROP_OVERWRITTEN) {
-    rv = NS_OK;
-  }
-  FlushChangesToTargetAttr();
-
-  return rv;
-}
-
-void
-nsSMILMappedAttribute::ClearAnimValue()
-{
-  RefPtr<nsIAtom> attrName = GetAttrNameAtom();
-  mElement->DeleteProperty(SMIL_MAPPED_ATTR_ANIMVAL, attrName);
-  FlushChangesToTargetAttr();
-}
-
-void
-nsSMILMappedAttribute::FlushChangesToTargetAttr() const
-{
-  // Clear animated content-style-rule
-  mElement->DeleteProperty(SMIL_MAPPED_ATTR_ANIMVAL,
-                           SMIL_MAPPED_ATTR_STYLEDECL_ATOM);
-  nsIDocument* doc = mElement->GetUncomposedDoc();
-
-  // Request animation restyle
-  if (doc) {
-    nsIPresShell* shell = doc->GetShell();
-    if (shell) {
-      shell->RestyleForAnimation(mElement, eRestyle_Self);
-    }
-  }
-}
-
-already_AddRefed<nsIAtom>
-nsSMILMappedAttribute::GetAttrNameAtom() const
-{
-  return NS_Atomize(nsCSSProps::GetStringValue(mPropID));
-}

deleted file mode 100644
--- a/dom/smil/nsSMILMappedAttribute.h
+++ /dev/null
@@ -1,56 +0,0 @@
-/* -*- 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/. */
-
-/* representation of a SMIL-animatable mapped attribute on an element */
-
-#ifndef NS_SMILMAPPEDATTRIBUTE_H_
-#define NS_SMILMAPPEDATTRIBUTE_H_
-
-#include "mozilla/Attributes.h"
-#include "nsSMILCSSProperty.h"
-
-/* We'll use the empty-string atom |nsGkAtoms::_empty| as the key for storing an
- * element's animated content style declaration in its Property Table, under the
- * property-category SMIL_MAPPED_ATTR_ANIMVAL.  Everything else stored in that
- * category is keyed off of the XML attribute name, so the empty string is a
- * good "reserved" key to use for storing the style rule (since XML attributes
- * all have nonempty names).
- */
-#define SMIL_MAPPED_ATTR_STYLEDECL_ATOM nsGkAtoms::_empty
-
-/**
- * nsSMILMappedAttribute: Implements the nsISMILAttr interface for SMIL
- * animations whose targets are attributes that map to CSS properties.  An
- * instance of this class represents a particular animation-targeted mapped
- * attribute on a particular element.
- */
-class nsSMILMappedAttribute : public nsSMILCSSProperty {
-public:
-  /**
-   * Constructs a new nsSMILMappedAttribute.
-   *
-   * @param  aPropID   The CSS property for the mapped attribute we're
-   *                   interested in animating.
-   * @param  aElement  The element whose attribute is being animated.
-   */
-  nsSMILMappedAttribute(nsCSSPropertyID aPropID, mozilla::dom::Element* aElement) :
-    nsSMILCSSProperty(aPropID, aElement) {}
-
-  // nsISMILAttr methods
-  virtual nsresult ValueFromString(const nsAString& aStr,
-                                   const mozilla::dom::SVGAnimationElement* aSrcElement,
-                                   nsSMILValue& aValue,
-                                   bool& aPreventCachingOfSandwich) const override;
-  virtual nsSMILValue GetBaseValue() const override;
-  virtual nsresult    SetAnimValue(const nsSMILValue& aValue) override;
-  virtual void        ClearAnimValue() override;
-
-protected:
-  // Helper Methods
-  void FlushChangesToTargetAttr() const;
-  already_AddRefed<nsIAtom> GetAttrNameAtom() const;
-};
-#endif // NS_SMILMAPPEDATTRIBUTE_H_

--- a/dom/smil/nsSMILTimeContainer.cpp
+++ b/dom/smil/nsSMILTimeContainer.cpp
@@ -6,16 +6,18 @@
 
 #include "nsSMILTimeContainer.h"
 #include "nsSMILTimeValue.h"
 #include "nsSMILTimedElement.h"
 #include <algorithm>
 
 #include "mozilla/AutoRestore.h"
 
+using namespace mozilla;
+
 nsSMILTimeContainer::nsSMILTimeContainer()
 :
   mParent(nullptr),
   mCurrentTime(0L),
   mParentOffset(0L),
   mPauseStart(0L),
   mNeedsPauseSample(false),
   mNeedsRewind(false),

--- a/dom/svg/nsSVGElement.cpp
+++ b/dom/svg/nsSVGElement.cpp
@@ -42,17 +42,16 @@
 #include "SVGAnimatedLengthList.h"
 #include "SVGAnimatedPointList.h"
 #include "SVGAnimatedPathSegList.h"
 #include "SVGContentUtils.h"
 #include "SVGGeometryElement.h"
 #include "nsIFrame.h"
 #include "nsQueryObject.h"
 #include <stdarg.h>
-#include "nsSMILMappedAttribute.h"
 #include "SVGMotionSMILAttr.h"
 #include "nsAttrValueOrString.h"
 #include "nsSMILAnimationController.h"
 #include "mozilla/dom/SVGElementBinding.h"
 #include "mozilla/DeclarationBlock.h"
 #include "mozilla/DeclarationBlockInlines.h"
 #include "mozilla/Unused.h"
 #include "mozilla/RestyleManager.h"
@@ -929,44 +928,16 @@ nsSVGElement::WalkContentStyleRules(nsRu
     css::Declaration* declaration = mContentDeclarationBlock->AsGecko();
     declaration->SetImmutable();
     aRuleWalker->Forward(declaration);
   }
 
   return NS_OK;
 }
 
-void
-nsSVGElement::WalkAnimatedContentStyleRules(nsRuleWalker* aRuleWalker)
-{
-  // Update & walk the animated content style rule, to include style from
-  // animated mapped attributes.  But first, get nsPresContext to check
-  // whether this is a "no-animation restyle". (This should match the check
-  // in nsHTMLCSSStyleSheet::RulesMatching(), where we determine whether to
-  // apply the SMILOverrideStyle.)
-  RestyleManager* restyleManager =
-    aRuleWalker->PresContext()->RestyleManager();
-  MOZ_ASSERT(restyleManager->IsGecko(),
-             "stylo: Servo-backed style system should not be calling "
-             "WalkAnimatedContentStyleRules");
-  if (!restyleManager->AsGecko()->SkipAnimationRules()) {
-    // update/walk the animated content style rule.
-    DeclarationBlock* animContentDeclBlock = GetAnimatedContentDeclarationBlock();
-    if (!animContentDeclBlock) {
-      UpdateAnimatedContentDeclarationBlock();
-      animContentDeclBlock = GetAnimatedContentDeclarationBlock();
-    }
-    if (animContentDeclBlock) {
-      css::Declaration* declaration = animContentDeclBlock->AsGecko();
-      declaration->SetImmutable();
-      aRuleWalker->Forward(declaration);
-    }
-  }
-}
-
 NS_IMETHODIMP_(bool)
 nsSVGElement::IsAttributeMapped(const nsIAtom* name) const
 {
   if (name == nsGkAtoms::lang) {
     return true;
   }
   return nsSVGElementBase::IsAttributeMapped(name);
 }
@@ -1376,95 +1347,16 @@ nsSVGElement::UpdateContentDeclarationBl
 }
 
 const DeclarationBlock*
 nsSVGElement::GetContentDeclarationBlock() const
 {
   return mContentDeclarationBlock;
 }
 
-static void
-ParseMappedAttrAnimValueCallback(void*    aObject,
-                                 nsIAtom* aPropertyName,
-                                 void*    aPropertyValue,
-                                 void*    aData)
-{
-  MOZ_ASSERT(aPropertyName != SMIL_MAPPED_ATTR_STYLEDECL_ATOM,
-             "animated content style rule should have been removed "
-             "from properties table already (we're rebuilding it now)");
-
-  MappedAttrParser* mappedAttrParser = static_cast<MappedAttrParser*>(aData);
-  MOZ_ASSERT(mappedAttrParser, "parser should be non-null");
-
-  nsStringBuffer* animValBuf = static_cast<nsStringBuffer*>(aPropertyValue);
-  MOZ_ASSERT(animValBuf, "animated value should be non-null");
-
-  nsString animValStr;
-  nsContentUtils::PopulateStringFromStringBuffer(animValBuf, animValStr);
-
-  mappedAttrParser->ParseMappedAttrValue(aPropertyName, animValStr);
-}
-
-// Callback for freeing animated content decl block, in property table.
-static void
-ReleaseDeclBlock(void*    aObject,       /* unused */
-                 nsIAtom* aPropertyName,
-                 void*    aPropertyValue,
-                 void*    aData          /* unused */)
-{
-  MOZ_ASSERT(aPropertyName == SMIL_MAPPED_ATTR_STYLEDECL_ATOM,
-             "unexpected property name, for animated content style rule");
-  auto decl = static_cast<DeclarationBlock*>(aPropertyValue);
-  MOZ_ASSERT(decl, "unexpected null decl");
-  decl->Release();
-}
-
-void
-nsSVGElement::UpdateAnimatedContentDeclarationBlock()
-{
-  MOZ_ASSERT(!GetAnimatedContentDeclarationBlock(),
-             "Animated content declaration block already set");
-
-  nsIDocument* doc = OwnerDoc();
-  if (!doc) {
-    NS_ERROR("SVG element without owner document");
-    return;
-  }
-
-  // FIXME (bug 1342557): Support SMIL in Servo
-  MappedAttrParser mappedAttrParser(doc->CSSLoader(), doc->GetDocumentURI(),
-                                    GetBaseURI(), this, StyleBackendType::Gecko);
-  doc->PropertyTable(SMIL_MAPPED_ATTR_ANIMVAL)->
-    Enumerate(this, ParseMappedAttrAnimValueCallback, &mappedAttrParser);
- 
-  RefPtr<DeclarationBlock> animContentDeclBlock =
-    mappedAttrParser.GetDeclarationBlock();
-
-  if (animContentDeclBlock) {
-#ifdef DEBUG
-    nsresult rv =
-#endif
-      SetProperty(SMIL_MAPPED_ATTR_ANIMVAL,
-                  SMIL_MAPPED_ATTR_STYLEDECL_ATOM,
-                  animContentDeclBlock.forget().take(),
-                  ReleaseDeclBlock);
-    MOZ_ASSERT(rv == NS_OK,
-               "SetProperty failed (or overwrote something)");
-  }
-}
-
-DeclarationBlock*
-nsSVGElement::GetAnimatedContentDeclarationBlock()
-{
-  return
-    static_cast<DeclarationBlock*>(GetProperty(SMIL_MAPPED_ATTR_ANIMVAL,
-                                               SMIL_MAPPED_ATTR_STYLEDECL_ATOM,
-                                               nullptr));
-}
-
 /**
  * Helper methods for the type-specific WillChangeXXX methods.
  *
  * This method sends out appropriate pre-change notifications so that selector
  * restyles (e.g. due to changes that cause |elem[attr="val"]| to start/stop
  * matching) work, and it returns an nsAttrValue that _may_ contain the
  * attribute's pre-change value.
  *

--- a/dom/svg/nsSVGElement.h
+++ b/dom/svg/nsSVGElement.h
@@ -352,19 +352,16 @@ protected:
   virtual nsresult AfterSetAttr(int32_t aNamespaceID, nsIAtom* aName,
                                 const nsAttrValue* aValue, bool aNotify) override;
   virtual bool ParseAttribute(int32_t aNamespaceID, nsIAtom* aAttribute,
                                 const nsAString& aValue, nsAttrValue& aResult) override;
   static nsresult ReportAttributeParseFailure(nsIDocument* aDocument,
                                               nsIAtom* aAttribute,
                                               const nsAString& aValue);
 
-  void UpdateAnimatedContentDeclarationBlock();
-  mozilla::DeclarationBlock* GetAnimatedContentDeclarationBlock();
-
   nsAttrValue WillChangeValue(nsIAtom* aName);
   // aNewValue is set to the old value. This value may be invalid if
   // !StoresOwnData.
   void DidChangeValue(nsIAtom* aName, const nsAttrValue& aEmptyOrOldValue,
                       nsAttrValue& aNewValue);
   void MaybeSerializeAttrBeforeRemoval(nsIAtom* aName, bool aNotify);
 
   static nsIAtom* GetEventNameForAttr(nsIAtom* aAttr);