diff --git a/dom/animation/AnimationPerformanceWarning.h b/dom/animation/AnimationPerformanceWarning.h
index a1ac35cba0..582c48cc3f 100644
--- a/dom/animation/AnimationPerformanceWarning.h
+++ b/dom/animation/AnimationPerformanceWarning.h
@@ -38,8 +38,6 @@ struct AnimationPerformanceWarning
                               std::initializer_list<int32_t> aParams)
     : mType(aType)
   {
-    // FIXME:  Once std::initializer_list::size() become a constexpr function,
-    // we should use static_assert here.
     MOZ_ASSERT(aParams.size() <= kMaxParamsForLocalization,
       "The length of parameters should be less than "
       "kMaxParamsForLocalization");
@@ -49,11 +47,11 @@ struct AnimationPerformanceWarning
   // Maximum number of parameters passed to
   // nsContentUtils::FormatLocalizedString to localize warning messages.
   //
-  // NOTE: This constexpr can't be forward declared, so if you want to use
+  // NOTE: This can't be forward declared, so if you want to use
   // this variable, please include this header file directly.
   // This value is the same as the limit of nsStringBundle::FormatString.
   // See the implementation of nsStringBundle::FormatString.
-  static constexpr uint8_t kMaxParamsForLocalization = 10;
+  static const uint8_t kMaxParamsForLocalization = 10;
 
   // Indicates why this property could not be animated on the compositor.
   Type mType;
diff --git a/dom/base/ResizeObserver.cpp b/dom/base/ResizeObserver.cpp
new file mode 100644
index 0000000000..37d940c2b3
--- /dev/null
+++ b/dom/base/ResizeObserver.cpp
@@ -0,0 +1,304 @@
+/* -*- 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/. */
+
+#include "mozilla/dom/ResizeObserver.h"
+
+#include "mozilla/dom/DOMRect.h"
+#include "nsContentUtils.h"
+#include "nsIFrame.h"
+#include "nsSVGUtils.h"
+
+namespace mozilla {
+namespace dom {
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ResizeObserver)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ResizeObserver)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ResizeObserver)
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(ResizeObserver)
+
+NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(ResizeObserver)
+  NS_IMPL_CYCLE_COLLECTION_TRACE_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_TRACE_END
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(ResizeObserver)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mOwner)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mCallback)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mObservationMap)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(ResizeObserver)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mOwner)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mCallback)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mObservationMap)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+already_AddRefed<ResizeObserver>
+ResizeObserver::Constructor(const GlobalObject& aGlobal,
+                            ResizeObserverCallback& aCb,
+                            ErrorResult& aRv)
+{
+  nsCOMPtr<nsPIDOMWindowInner> window =
+    do_QueryInterface(aGlobal.GetAsSupports());
+
+  if (!window) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    return nullptr;
+  }
+
+  nsCOMPtr<nsIDocument> document = window->GetExtantDoc();
+
+  if (!document) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    return nullptr;
+  }
+
+  RefPtr<ResizeObserver> observer = new ResizeObserver(window.forget(), aCb);
+  document->AddResizeObserver(observer);
+
+  return observer.forget();
+}
+
+void
+ResizeObserver::Observe(Element* aTarget,
+                        ErrorResult& aRv)
+{
+  if (!aTarget) {
+    aRv.Throw(NS_ERROR_DOM_NOT_FOUND_ERR);
+    return;
+  }
+
+  RefPtr<ResizeObservation> observation;
+
+  if (!mObservationMap.Get(aTarget, getter_AddRefs(observation))) {
+    observation = new ResizeObservation(this, aTarget);
+
+    mObservationMap.Put(aTarget, observation);
+    mObservationList.insertBack(observation);
+
+    // Per the spec, we need to trigger notification in event loop that
+    // contains ResizeObserver observe call even when resize/reflow does
+    // not happen.
+    aTarget->OwnerDoc()->ScheduleResizeObserversNotification();
+  }
+}
+
+void
+ResizeObserver::Unobserve(Element* aTarget,
+                          ErrorResult& aRv)
+{
+  if (!aTarget) {
+    aRv.Throw(NS_ERROR_DOM_NOT_FOUND_ERR);
+    return;
+  }
+
+  RefPtr<ResizeObservation> observation;
+
+  if (mObservationMap.Get(aTarget, getter_AddRefs(observation))) {
+    mObservationMap.Remove(aTarget);
+
+    MOZ_ASSERT(!mObservationList.isEmpty(),
+               "If ResizeObservation found for an element, observation list "
+               "must be not empty.");
+
+    observation->remove();
+  }
+}
+
+void
+ResizeObserver::Disconnect()
+{
+  mObservationMap.Clear();
+  mObservationList.clear();
+  mActiveTargets.Clear();
+}
+
+void
+ResizeObserver::GatherActiveObservations(uint32_t aDepth)
+{
+  mActiveTargets.Clear();
+  mHasSkippedTargets = false;
+
+  for (auto observation : mObservationList) {
+    if (observation->IsActive()) {
+      uint32_t targetDepth =
+        nsContentUtils::GetNodeDepth(observation->Target());
+
+      if (targetDepth > aDepth) {
+        mActiveTargets.AppendElement(observation);
+      } else {
+        mHasSkippedTargets = true;
+      }
+    }
+  }
+}
+
+bool
+ResizeObserver::HasActiveObservations() const
+{
+  return !mActiveTargets.IsEmpty();
+}
+
+bool
+ResizeObserver::HasSkippedObservations() const
+{
+  return mHasSkippedTargets;
+}
+
+uint32_t
+ResizeObserver::BroadcastActiveObservations()
+{
+  uint32_t shallowestTargetDepth = UINT32_MAX;
+
+  if (HasActiveObservations()) {
+    Sequence<OwningNonNull<ResizeObserverEntry>> entries;
+
+    for (auto observation : mActiveTargets) {
+      RefPtr<ResizeObserverEntry> entry =
+        new ResizeObserverEntry(this, observation->Target());
+
+      nsRect rect = observation->GetTargetRect();
+      entry->SetContentRect(rect);
+
+      if (!entries.AppendElement(entry.forget(), fallible)) {
+        // Out of memory.
+        break;
+      }
+
+      // Sync the broadcast size of observation so the next size inspection
+      // will be based on the updated size from last delivered observations.
+      observation->UpdateBroadcastSize(rect);
+
+      uint32_t targetDepth =
+        nsContentUtils::GetNodeDepth(observation->Target());
+
+      if (targetDepth < shallowestTargetDepth) {
+        shallowestTargetDepth = targetDepth;
+      }
+    }
+
+    mCallback->Call(this, entries, *this);
+    mActiveTargets.Clear();
+    mHasSkippedTargets = false;
+  }
+
+  return shallowestTargetDepth;
+}
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ResizeObserverEntry)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ResizeObserverEntry)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ResizeObserverEntry)
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(ResizeObserverEntry,
+                                      mTarget, mContentRect,
+                                      mOwner)
+
+already_AddRefed<ResizeObserverEntry>
+ResizeObserverEntry::Constructor(const GlobalObject& aGlobal,
+                                 Element* aTarget,
+                                 ErrorResult& aRv)
+{
+  RefPtr<ResizeObserverEntry> observerEntry =
+    new ResizeObserverEntry(aGlobal.GetAsSupports(), aTarget);
+  return observerEntry.forget();
+}
+
+void
+ResizeObserverEntry::SetContentRect(nsRect aRect)
+{
+  RefPtr<DOMRect> contentRect = new DOMRect(mTarget);
+  nsIFrame* frame = mTarget->GetPrimaryFrame();
+
+  if (frame) {
+    nsMargin padding = frame->GetUsedPadding();
+
+    // Per the spec, we need to include padding in contentRect of
+    // ResizeObserverEntry.
+    aRect.x = padding.left;
+    aRect.y = padding.top;
+  }
+
+  contentRect->SetLayoutRect(aRect);
+  mContentRect = contentRect.forget();
+}
+
+ResizeObserverEntry::~ResizeObserverEntry()
+{
+}
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ResizeObservation)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ResizeObservation)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ResizeObservation)
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(ResizeObservation,
+                                      mTarget, mOwner)
+
+already_AddRefed<ResizeObservation>
+ResizeObservation::Constructor(const GlobalObject& aGlobal,
+                               Element* aTarget,
+                               ErrorResult& aRv)
+{
+  RefPtr<ResizeObservation> observation =
+    new ResizeObservation(aGlobal.GetAsSupports(), aTarget);
+  return observation.forget();
+}
+
+bool
+ResizeObservation::IsActive() const
+{
+  nsRect rect = GetTargetRect();
+  return (rect.width != mBroadcastWidth || rect.height != mBroadcastHeight);
+}
+
+void
+ResizeObservation::UpdateBroadcastSize(nsRect aRect)
+{
+  mBroadcastWidth = aRect.width;
+  mBroadcastHeight = aRect.height;
+}
+
+nsRect
+ResizeObservation::GetTargetRect() const
+{
+  nsRect rect;
+  nsIFrame* frame = mTarget->GetPrimaryFrame();
+
+  if (frame) {
+    if (mTarget->IsSVGElement()) {
+      gfxRect bbox = nsSVGUtils::GetBBox(frame);
+      rect.width = NSFloatPixelsToAppUnits(bbox.width, AppUnitsPerCSSPixel());
+      rect.height = NSFloatPixelsToAppUnits(bbox.height, AppUnitsPerCSSPixel());
+    } else {
+      // Per the spec, non-replaced inline Elements will always have an empty
+      // content rect.
+      if (frame->IsFrameOfType(nsIFrame::eReplaced) ||
+          !frame->IsFrameOfType(nsIFrame::eLineParticipant)) {
+        rect = frame->GetContentRectRelativeToSelf();
+      }
+    }
+  }
+
+  return rect;
+}
+
+ResizeObservation::~ResizeObservation()
+{
+}
+
+} // namespace dom
+} // namespace mozilla
diff --git a/dom/base/ResizeObserver.h b/dom/base/ResizeObserver.h
new file mode 100644
index 0000000000..2f56c580f4
--- /dev/null
+++ b/dom/base/ResizeObserver.h
@@ -0,0 +1,254 @@
+/* -*- 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_dom_ResizeObserver_h
+#define mozilla_dom_ResizeObserver_h
+
+#include "mozilla/dom/ResizeObserverBinding.h"
+
+namespace mozilla {
+namespace dom {
+
+/**
+ * ResizeObserver interfaces and algorithms are based on
+ * https://wicg.github.io/ResizeObserver/#api
+ */
+class ResizeObserver final
+  : public nsISupports
+  , public nsWrapperCache
+{
+public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(ResizeObserver)
+
+  ResizeObserver(already_AddRefed<nsPIDOMWindowInner>&& aOwner,
+                 ResizeObserverCallback& aCb)
+    : mOwner(aOwner)
+    , mCallback(&aCb)
+  {
+    MOZ_ASSERT(mOwner, "Need a non-null owner window");
+  }
+
+  static already_AddRefed<ResizeObserver>
+  Constructor(const GlobalObject& aGlobal,
+              ResizeObserverCallback& aCb,
+              ErrorResult& aRv);
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override
+  {
+    return ResizeObserverBinding::Wrap(aCx, this, aGivenProto);
+  }
+
+  nsISupports* GetParentObject() const
+  {
+    return mOwner;
+  }
+
+  void Observe(Element* aTarget, ErrorResult& aRv);
+
+  void Unobserve(Element* aTarget, ErrorResult& aRv);
+
+  void Disconnect();
+
+  /*
+   * Gather all observations which have an observed target with size changed
+   * since last BroadcastActiveObservations() in this ResizeObserver.
+   * An observation will be skipped if the depth of its observed target is less
+   * or equal than aDepth. All gathered observations will be added to
+   * mActiveTargets.
+  */
+  void GatherActiveObservations(uint32_t aDepth);
+
+  /*
+   * Returns whether this ResizeObserver has any active observations
+   * since last GatherActiveObservations().
+  */
+  bool HasActiveObservations() const;
+
+  /*
+   * Returns whether this ResizeObserver has any skipped observations
+   * since last GatherActiveObservations().
+  */
+  bool HasSkippedObservations() const;
+
+  /*
+   * Deliver the callback function in JavaScript for all active observations
+   * and pass the sequence of ResizeObserverEntry so JavaScript can access them.
+   * The broadcast size of observations will be updated and mActiveTargets will
+   * be cleared. It also returns the shallowest depth of elements from active
+   * observations or UINT32_MAX if there is no any active observations.
+  */
+  uint32_t BroadcastActiveObservations();
+
+protected:
+  ~ResizeObserver()
+  {
+    mObservationList.clear();
+  }
+
+  nsCOMPtr<nsPIDOMWindowInner> mOwner;
+  RefPtr<ResizeObserverCallback> mCallback;
+  nsTArray<RefPtr<ResizeObservation>> mActiveTargets;
+  bool mHasSkippedTargets;
+
+  // Combination of HashTable and LinkedList so we can iterate through
+  // the elements of HashTable in order of insertion time.
+  // Will be nice if we have our own data structure for this in the future.
+  nsRefPtrHashtable<nsPtrHashKey<Element>, ResizeObservation> mObservationMap;
+  LinkedList<ResizeObservation> mObservationList;
+};
+
+/**
+ * ResizeObserverEntry is the entry that contains the information for observed
+ * elements. This object is the one that visible to JavaScript in callback
+ * function that is fired by ResizeObserver.
+ */
+class ResizeObserverEntry final
+  : public nsISupports
+  , public nsWrapperCache
+{
+public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(ResizeObserverEntry)
+
+  ResizeObserverEntry(nsISupports* aOwner, Element* aTarget)
+    : mOwner(aOwner)
+    , mTarget(aTarget)
+  {
+    MOZ_ASSERT(mOwner, "Need a non-null owner");
+    MOZ_ASSERT(mTarget, "Need a non-null target element");
+  }
+
+  static already_AddRefed<ResizeObserverEntry>
+  Constructor(const GlobalObject& aGlobal,
+              Element* aTarget,
+              ErrorResult& aRv);
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override
+  {
+    return ResizeObserverEntryBinding::Wrap(aCx, this,
+                                            aGivenProto);
+  }
+
+  nsISupports* GetParentObject() const
+  {
+    return mOwner;
+  }
+
+  Element* Target() const
+  {
+    return mTarget;
+  }
+
+  /*
+   * Returns the DOMRectReadOnly of target's content rect so it can be
+   * accessed from JavaScript in callback function of ResizeObserver.
+  */
+  DOMRectReadOnly* GetContentRect() const
+  {
+    return mContentRect;
+  }
+
+  void SetContentRect(nsRect aRect);
+
+protected:
+  ~ResizeObserverEntry();
+
+  nsCOMPtr<nsISupports> mOwner;
+  nsCOMPtr<Element> mTarget;
+  RefPtr<DOMRectReadOnly> mContentRect;
+};
+
+/**
+ * We use ResizeObservation to store and sync the size information of one
+ * observed element so we can decide whether an observation should be fired
+ * or not.
+ */
+class ResizeObservation final
+  : public nsISupports
+  , public nsWrapperCache
+  , public LinkedListElement<ResizeObservation>
+{
+public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(ResizeObservation)
+
+  ResizeObservation(nsISupports* aOwner, Element* aTarget)
+    : mOwner(aOwner)
+    , mTarget(aTarget)
+    , mBroadcastWidth(0)
+    , mBroadcastHeight(0)
+  {
+    MOZ_ASSERT(mOwner, "Need a non-null owner");
+    MOZ_ASSERT(mTarget, "Need a non-null target element");
+  }
+
+  static already_AddRefed<ResizeObservation>
+  Constructor(const GlobalObject& aGlobal,
+              Element* aTarget,
+              ErrorResult& aRv);
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override
+  {
+    return ResizeObservationBinding::Wrap(aCx, this, aGivenProto);
+  }
+
+  nsISupports* GetParentObject() const
+  {
+    return mOwner;
+  }
+
+  Element* Target() const
+  {
+    return mTarget;
+  }
+
+  nscoord BroadcastWidth() const
+  {
+    return mBroadcastWidth;
+  }
+
+  nscoord BroadcastHeight() const
+  {
+    return mBroadcastHeight;
+  }
+
+  /*
+   * Returns whether the observed target element size differs from current
+   * BroadcastWidth and BroadcastHeight
+  */
+  bool IsActive() const;
+
+  /*
+   * Update current BroadcastWidth and BroadcastHeight with size from aRect.
+  */
+  void UpdateBroadcastSize(nsRect aRect);
+
+  /*
+   * Returns the target's rect in the form of nsRect.
+   * If the target is SVG, width and height are determined from bounding box.
+  */
+  nsRect GetTargetRect() const;
+
+protected:
+  ~ResizeObservation();
+
+  nsCOMPtr<nsISupports> mOwner;
+  nsCOMPtr<Element> mTarget;
+
+  // Broadcast width and broadcast height are the latest recorded size
+  // of observed target.
+  nscoord mBroadcastWidth;
+  nscoord mBroadcastHeight;
+};
+
+} // namespace dom
+} // namespace mozilla
+
+#endif // mozilla_dom_ResizeObserver_h
+
diff --git a/dom/base/ResizeObserverController.cpp b/dom/base/ResizeObserverController.cpp
new file mode 100644
index 0000000000..924bba10d1
--- /dev/null
+++ b/dom/base/ResizeObserverController.cpp
@@ -0,0 +1,234 @@
+/* -*- 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/ResizeObserverController.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/ErrorEvent.h"
+#include "nsIPresShell.h"
+#include "nsPresContext.h"
+
+namespace mozilla {
+namespace dom {
+
+void
+ResizeObserverNotificationHelper::WillRefresh(TimeStamp aTime)
+{
+  MOZ_ASSERT(mOwner, "Why is mOwner already dead when this RefreshObserver is still registered?");
+  if (mOwner) {
+    mOwner->Notify();
+  }
+}
+
+nsRefreshDriver*
+ResizeObserverNotificationHelper::GetRefreshDriver() const
+{
+  nsIPresShell* presShell = mOwner->GetShell();
+  if (MOZ_UNLIKELY(!presShell)) {
+    return nullptr;
+  }
+
+  nsPresContext* presContext = presShell->GetPresContext();
+  if (MOZ_UNLIKELY(!presContext)) {
+    return nullptr;
+  }
+
+  return presContext->RefreshDriver();
+}
+
+void
+ResizeObserverNotificationHelper::Register()
+{
+  if (mRegistered) {
+    return;
+  }
+
+  nsRefreshDriver* refreshDriver = GetRefreshDriver();
+  if (!refreshDriver) {
+    // We maybe navigating away from this page or currently in an iframe with
+    // display: none. Just abort the Register(), no need to do notification.
+    return;
+  }
+
+  refreshDriver->AddRefreshObserver(this, Flush_Display);
+  mRegistered = true;
+}
+
+void
+ResizeObserverNotificationHelper::Unregister()
+{
+  if (!mRegistered) {
+    return;
+  }
+
+  nsRefreshDriver* refreshDriver = GetRefreshDriver();
+  if (!refreshDriver) {
+    // We can't access RefreshDriver now. Just abort the Unregister().
+    return;
+  }
+
+  refreshDriver->RemoveRefreshObserver(this, Flush_Display);
+  mRegistered = false;
+}
+
+void
+ResizeObserverNotificationHelper::Disconnect()
+{
+  Unregister();
+  // Our owner is dying. Clear our pointer to it, in case we outlive it.
+  mOwner = nullptr;
+}
+
+ResizeObserverNotificationHelper::~ResizeObserverNotificationHelper()
+{
+  Unregister();
+}
+
+void
+ResizeObserverController::Traverse(nsCycleCollectionTraversalCallback& aCb)
+{
+  ImplCycleCollectionTraverse(aCb, mResizeObservers, "mResizeObservers");
+}
+
+void
+ResizeObserverController::Unlink()
+{
+  mResizeObservers.Clear();
+}
+
+void
+ResizeObserverController::AddResizeObserver(ResizeObserver* aObserver)
+{
+  MOZ_ASSERT(aObserver, "AddResizeObserver() should never be called with "
+                        "a null parameter");
+  mResizeObservers.AppendElement(aObserver);
+}
+
+void
+ResizeObserverController::Notify()
+{
+  if (mResizeObservers.IsEmpty()) {
+    return;
+  }
+
+  uint32_t shallowestTargetDepth = 0;
+
+  GatherAllActiveObservations(shallowestTargetDepth);
+
+  while (HasAnyActiveObservations()) {
+    DebugOnly<uint32_t> oldShallowestTargetDepth = shallowestTargetDepth;
+    shallowestTargetDepth = BroadcastAllActiveObservations();
+    NS_ASSERTION(oldShallowestTargetDepth < shallowestTargetDepth,
+                 "shallowestTargetDepth should be getting strictly deeper");
+
+    // Flush layout, so that any callback functions' style changes / resizes
+    // get a chance to take effect.
+    mDocument->FlushPendingNotifications(Flush_Layout);
+
+    // To avoid infinite resize loop, we only gather all active observations
+    // that have the depth of observed target element more than current
+    // shallowestTargetDepth.
+    GatherAllActiveObservations(shallowestTargetDepth);
+  }
+
+  mResizeObserverNotificationHelper->Unregister();
+
+  // Per spec, we deliver an error if the document has any skipped observations.
+  if (HasAnySkippedObservations()) {
+    RootedDictionary<ErrorEventInit> init(RootingCx());
+
+    init.mMessage.AssignLiteral("ResizeObserver loop completed with undelivered"
+                                " notifications.");
+    init.mCancelable = true;
+    init.mBubbles = true;
+
+    nsEventStatus status = nsEventStatus_eIgnore;
+
+    nsCOMPtr<nsPIDOMWindowInner> window =
+      mDocument->GetWindow()->GetCurrentInnerWindow();
+
+    if (window) {
+      nsCOMPtr<nsIScriptGlobalObject> sgo = do_QueryInterface(window);
+      MOZ_ASSERT(sgo);
+
+      if (NS_WARN_IF(NS_FAILED(sgo->HandleScriptError(init, &status)))) {
+        status = nsEventStatus_eIgnore;
+      }
+    } else {
+      // We don't fire error events at any global for non-window JS on the main
+      // thread.
+    }
+
+    // We need to deliver pending notifications in next cycle.
+    ScheduleNotification();
+  }
+}
+
+void
+ResizeObserverController::GatherAllActiveObservations(uint32_t aDepth)
+{
+  for (auto observer : mResizeObservers) {
+    observer->GatherActiveObservations(aDepth);
+  }
+}
+
+uint32_t
+ResizeObserverController::BroadcastAllActiveObservations()
+{
+  uint32_t shallowestTargetDepth = UINT32_MAX;
+
+  for (auto observer : mResizeObservers) {
+
+    uint32_t targetDepth = observer->BroadcastActiveObservations();
+
+    if (targetDepth < shallowestTargetDepth) {
+      shallowestTargetDepth = targetDepth;
+    }
+  }
+
+  return shallowestTargetDepth;
+}
+
+bool
+ResizeObserverController::HasAnyActiveObservations() const
+{
+  for (auto observer : mResizeObservers) {
+    if (observer->HasActiveObservations()) {
+      return true;
+    }
+  }
+  return false;
+}
+
+bool
+ResizeObserverController::HasAnySkippedObservations() const
+{
+  for (auto observer : mResizeObservers) {
+    if (observer->HasSkippedObservations()) {
+      return true;
+    }
+  }
+  return false;
+}
+
+void
+ResizeObserverController::ScheduleNotification()
+{
+  mResizeObserverNotificationHelper->Register();
+}
+
+nsIPresShell*
+ResizeObserverController::GetShell() const
+{
+  return mDocument->GetShell();
+}
+
+ResizeObserverController::~ResizeObserverController()
+{
+  mResizeObserverNotificationHelper->Disconnect();
+}
+
+} // namespace dom
+} // namespace mozilla
diff --git a/dom/base/ResizeObserverController.h b/dom/base/ResizeObserverController.h
new file mode 100644
index 0000000000..a775115874
--- /dev/null
+++ b/dom/base/ResizeObserverController.h
@@ -0,0 +1,129 @@
+/* -*- 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_ResizeObserverController_h
+#define mozilla_dom_ResizeObserverController_h
+
+#include "mozilla/dom/ResizeObserver.h"
+#include "mozilla/TimeStamp.h"
+#include "nsRefreshDriver.h"
+
+namespace mozilla {
+namespace dom {
+
+class ResizeObserverController;
+
+/*
+ * ResizeObserverNotificationHelper will trigger ResizeObserver notifications
+ * by registering with the Refresh Driver.
+*/
+class ResizeObserverNotificationHelper final : public nsARefreshObserver
+{
+public:
+  NS_INLINE_DECL_REFCOUNTING(ResizeObserverNotificationHelper, override)
+
+  explicit ResizeObserverNotificationHelper(ResizeObserverController* aOwner)
+    : mOwner(aOwner)
+    , mRegistered(false)
+  {
+    MOZ_ASSERT(mOwner, "Need a non-null owner");
+  }
+
+  void WillRefresh(TimeStamp aTime) override;
+
+  nsRefreshDriver* GetRefreshDriver() const;
+
+  void Register();
+
+  void Unregister();
+
+  void Disconnect();
+
+protected:
+  virtual ~ResizeObserverNotificationHelper();
+
+  ResizeObserverController* mOwner;
+  bool mRegistered;
+};
+
+/*
+ * ResizeObserverController contains the list of ResizeObservers and controls
+ * the flow of notification.
+*/
+class ResizeObserverController final
+{
+public:
+  explicit ResizeObserverController(nsIDocument* aDocument)
+    : mDocument(aDocument)
+    , mIsNotificationActive(false)
+  {
+    MOZ_ASSERT(mDocument, "Need a non-null document");
+    mResizeObserverNotificationHelper =
+      new ResizeObserverNotificationHelper(this);
+  }
+
+  // Methods for supporting cycle-collection
+  void Traverse(nsCycleCollectionTraversalCallback& aCb);
+  void Unlink();
+
+  void AddResizeObserver(ResizeObserver* aObserver);
+
+  /*
+   * Schedule the notification via ResizeObserverNotificationHelper refresh
+   * observer.
+  */
+  void ScheduleNotification();
+
+  /*
+   * Notify all ResizeObservers by gathering and broadcasting all active
+   * observations.
+  */
+  void Notify();
+
+  nsIPresShell* GetShell() const;
+
+  ~ResizeObserverController();
+
+private:
+  /*
+   * Calls GatherActiveObservations(aDepth) for all ResizeObservers in this
+   * controller. All observations in each ResizeObserver with element's depth
+   * more than aDepth will be gathered.
+  */
+  void GatherAllActiveObservations(uint32_t aDepth);
+
+  /*
+   * Calls BroadcastActiveObservations() for all ResizeObservers in this
+   * controller. It also returns the shallowest depth of observed target
+   * elements from all ResizeObserver or UINT32_MAX if there is no any
+   * active obsevations at all.
+  */
+  uint32_t BroadcastAllActiveObservations();
+
+  /*
+   * Returns whether there is any ResizeObserver that has active observations.
+  */
+  bool HasAnyActiveObservations() const;
+
+  /*
+   * Returns whether there is any ResizeObserver that has skipped observations.
+  */
+  bool HasAnySkippedObservations() const;
+
+protected:
+  // Raw pointer is OK because mDocument strongly owns us & hence must outlive
+  // us.
+  nsIDocument* const mDocument;
+
+  RefPtr<ResizeObserverNotificationHelper> mResizeObserverNotificationHelper;
+  nsTArray<RefPtr<ResizeObserver>> mResizeObservers;
+  bool mIsNotificationActive;
+};
+
+} // namespace dom
+} // namespace mozilla
+
+#endif // mozilla_dom_ResizeObserverController_h
diff --git a/dom/base/WindowNamedPropertiesHandler.h b/dom/base/WindowNamedPropertiesHandler.h
index 227d8c946d..cafeadb85e 100644
--- a/dom/base/WindowNamedPropertiesHandler.h
+++ b/dom/base/WindowNamedPropertiesHandler.h
@@ -15,7 +15,7 @@ namespace dom {
 class WindowNamedPropertiesHandler : public BaseDOMProxyHandler
 {
 public:
-  constexpr WindowNamedPropertiesHandler()
+  WindowNamedPropertiesHandler()
     : BaseDOMProxyHandler(nullptr, /* hasPrototype = */ true)
   {
   }
diff --git a/dom/base/moz.build b/dom/base/moz.build
index fe65453fe9..aff13bee0c 100755
--- a/dom/base/moz.build
+++ b/dom/base/moz.build
@@ -204,6 +204,8 @@ EXPORTS.mozilla.dom += [
     'PartialSHistory.h',
     'Pose.h',
     'ProcessGlobal.h',
+    'ResizeObserver.h',
+    'ResizeObserverController.h',
     'ResponsiveImageSelector.h',
     'SameProcessMessageQueue.h',
     'ScreenOrientation.h',
@@ -350,6 +352,8 @@ SOURCES += [
     'Pose.cpp',
     'PostMessageEvent.cpp',
     'ProcessGlobal.cpp',
+    'ResizeObserver.cpp',
+    'ResizeObserverController.cpp',
     'ResponsiveImageSelector.cpp',
     'SameProcessMessageQueue.cpp',
     'ScreenOrientation.cpp',
diff --git a/dom/base/nsAttrValue.h b/dom/base/nsAttrValue.h
index 23f61a614a..33ee91afd5 100644
--- a/dom/base/nsAttrValue.h
+++ b/dom/base/nsAttrValue.h
@@ -268,7 +268,7 @@ public:
     // EnumTable can be initialized either with an int16_t value
     // or a value of an enumeration type that can fit within an int16_t.
 
-    constexpr EnumTable(const char* aTag, int16_t aValue)
+    EnumTable(const char* aTag, int16_t aValue)
       : tag(aTag)
       , value(aValue)
     {
@@ -276,7 +276,7 @@ public:
 
     template<typename T,
              typename = typename std::enable_if<std::is_enum<T>::value>::type>
-    constexpr EnumTable(const char* aTag, T aValue)
+    EnumTable(const char* aTag, T aValue)
       : tag(aTag)
       , value(static_cast<int16_t>(aValue))
     {
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 3568ced90d..20b6beffa1 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -9843,3 +9843,17 @@ nsContentUtils::GetClosestNonNativeAnonymousAncestor(Element* aElement)
   }
   return e;
 }
+
+/* static */ uint32_t
+nsContentUtils::GetNodeDepth(nsINode* aNode)
+{
+  uint32_t depth = 1;
+
+  MOZ_ASSERT(aNode, "Node shouldn't be null");
+
+  while ((aNode = aNode->GetParentNode())) {
+    ++depth;
+  }
+
+  return depth;
+}
\ No newline at end of file
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 8cf105bb3e..db9558d06c 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -2763,6 +2763,14 @@ public:
   static bool
   IsCustomElementsEnabled() { return sIsCustomElementsEnabled; }
 
+  /**
+   * Returns the length of the parent-traversal path (in terms of the number of
+   * nodes) to an unparented/root node from aNode. An unparented/root node is
+   * considered to have a depth of 1, its children have a depth of 2, etc.
+   * aNode is expected to be non-null.
+   */
+  static uint32_t GetNodeDepth(nsINode* aNode);
+
 private:
   static bool InitializeEventTable();
 
diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index 76490e6b44..5bf89f26ae 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -1681,6 +1681,10 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INTERNAL(nsDocument)
       cb.NoteXPCOMChild(mql);
     }
   }
+
+  if (tmp->mResizeObserverController) {
+    tmp->mResizeObserverController->Traverse(cb);
+  }
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
 NS_IMPL_CYCLE_COLLECTION_CLASS(nsDocument)
@@ -1786,6 +1790,10 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsDocument)
   }
 
   tmp->mInUnlinkOrDeletion = false;
+
+  if (tmp->mResizeObserverController) {
+    tmp->mResizeObserverController->Unlink();
+  }
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
 nsresult
@@ -11823,6 +11831,24 @@ nsDocument::QuerySelectorAll(const nsAString& aSelector, nsIDOMNodeList **aRetur
   return nsINode::QuerySelectorAll(aSelector, aReturn);
 }
 
+void
+nsDocument::AddResizeObserver(ResizeObserver* aResizeObserver)
+{
+  if (!mResizeObserverController) {
+    mResizeObserverController = MakeUnique<ResizeObserverController>(this);
+  }
+
+  mResizeObserverController->AddResizeObserver(aResizeObserver);
+}
+
+void
+nsDocument::ScheduleResizeObserversNotification() const
+{
+  if (mResizeObserverController) {
+    mResizeObserverController->ScheduleNotification();
+  }
+}
+
 already_AddRefed<nsIDocument>
 nsIDocument::Constructor(const GlobalObject& aGlobal,
                          ErrorResult& rv)
diff --git a/dom/base/nsDocument.h b/dom/base/nsDocument.h
index 502ba0f133..017879b1fb 100644
--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -60,6 +60,7 @@
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/PendingAnimationTracker.h"
 #include "mozilla/dom/DOMImplementation.h"
+#include "mozilla/dom/ResizeObserverController.h"
 #include "mozilla/dom/ScriptLoader.h"
 #include "mozilla/dom/StyleSheetList.h"
 #include "nsDataHashtable.h"
@@ -1024,6 +1025,10 @@ public:
 
   virtual void UnblockDOMContentLoaded() override;
 
+  void AddResizeObserver(mozilla::dom::ResizeObserver* aResizeObserver) override;
+
+  void ScheduleResizeObserversNotification() const override;
+
 protected:
   friend class nsNodeUtils;
   friend class nsDocumentOnStack;
@@ -1160,6 +1165,9 @@ protected:
 
   nsTArray<nsIObserver*> mCharSetObservers;
 
+  mozilla::UniquePtr<mozilla::dom::ResizeObserverController>
+    mResizeObserverController;
+
   PLDHashTable *mSubDocuments;
 
   // Array of owning references to all children
diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index acd596a442..642681d910 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -956,7 +956,7 @@ NS_IMPL_CYCLE_COLLECTING_RELEASE(DialogValueHolder)
 class nsOuterWindowProxy : public js::Wrapper
 {
 public:
-  constexpr nsOuterWindowProxy() : js::Wrapper(0) { }
+  nsOuterWindowProxy() : js::Wrapper(0) { }
 
   virtual bool finalizeInBackground(const JS::Value& priv) const override {
     return false;
@@ -1407,7 +1407,7 @@ nsOuterWindowProxy::singleton;
 class nsChromeOuterWindowProxy : public nsOuterWindowProxy
 {
 public:
-  constexpr nsChromeOuterWindowProxy() : nsOuterWindowProxy() { }
+  nsChromeOuterWindowProxy() : nsOuterWindowProxy() { }
 
   virtual const char *className(JSContext *cx, JS::Handle<JSObject*> wrapper) const override;
 
diff --git a/dom/base/nsIDocument.h b/dom/base/nsIDocument.h
index 21cd0aaf7c..5684490aa5 100644
--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -153,6 +153,7 @@ class ProcessingInstruction;
 class Promise;
 class Selection;
 class ScriptLoader;
+class ResizeObserver;
 class StyleSheetList;
 class SVGDocument;
 class SVGSVGElement;
@@ -2853,6 +2854,10 @@ public:
 
   bool ModuleScriptsEnabled();
 
+  virtual void AddResizeObserver(mozilla::dom::ResizeObserver* aResizeObserver) = 0;
+
+  virtual void ScheduleResizeObserversNotification() const = 0;
+
 protected:
   bool GetUseCounter(mozilla::UseCounter aUseCounter)
   {
diff --git a/dom/base/nsJSEnvironment.cpp b/dom/base/nsJSEnvironment.cpp
index 605b1917fd..0411bee80a 100644
--- a/dom/base/nsJSEnvironment.cpp
+++ b/dom/base/nsJSEnvironment.cpp
@@ -1161,7 +1161,7 @@ TimeUntilNow(TimeStamp start)
 
 struct CycleCollectorStats
 {
-  constexpr CycleCollectorStats() :
+  CycleCollectorStats() :
     mMaxGCDuration(0), mRanSyncForgetSkippable(false), mSuspected(0),
     mMaxSkippableDuration(0), mMaxSliceTime(0), mMaxSliceTimeSinceClear(0),
     mTotalSliceTime(0), mAnyLockedOut(false), mExtraForgetSkippableCalls(0),
diff --git a/dom/bindings/BindingUtils.h b/dom/bindings/BindingUtils.h
index 356d3aa000..e97f29359b 100644
--- a/dom/bindings/BindingUtils.h
+++ b/dom/bindings/BindingUtils.h
@@ -3073,21 +3073,21 @@ class GetCCParticipant
 {
   // Helper for GetCCParticipant for classes that participate in CC.
   template<class U>
-  static constexpr nsCycleCollectionParticipant*
+  static nsCycleCollectionParticipant*
   GetHelper(int, typename U::NS_CYCLE_COLLECTION_INNERCLASS* dummy=nullptr)
   {
     return T::NS_CYCLE_COLLECTION_INNERCLASS::GetParticipant();
   }
   // Helper for GetCCParticipant for classes that don't participate in CC.
   template<class U>
-  static constexpr nsCycleCollectionParticipant*
+  static nsCycleCollectionParticipant*
   GetHelper(double)
   {
     return nullptr;
   }
 
 public:
-  static constexpr nsCycleCollectionParticipant*
+  static nsCycleCollectionParticipant*
   Get()
   {
     // Passing int() here will try to call the GetHelper that takes an int as
@@ -3102,7 +3102,7 @@ template<class T>
 class GetCCParticipant<T, true>
 {
 public:
-  static constexpr nsCycleCollectionParticipant*
+  static nsCycleCollectionParticipant*
   Get()
   {
     return nullptr;
@@ -3125,7 +3125,7 @@ EnumerateGlobal(JSContext* aCx, JS::Handle<JSObject*> aObj);
 template <class T>
 struct CreateGlobalOptions
 {
-  static constexpr ProtoAndIfaceCache::Kind ProtoAndIfaceCacheKind =
+  static const ProtoAndIfaceCache::Kind ProtoAndIfaceCacheKind =
     ProtoAndIfaceCache::NonWindowLike;
   static void TraceGlobal(JSTracer* aTrc, JSObject* aObj)
   {
@@ -3142,7 +3142,7 @@ struct CreateGlobalOptions
 template <>
 struct CreateGlobalOptions<nsGlobalWindow>
 {
-  static constexpr ProtoAndIfaceCache::Kind ProtoAndIfaceCacheKind =
+  static const ProtoAndIfaceCache::Kind ProtoAndIfaceCacheKind =
     ProtoAndIfaceCache::WindowLike;
   static void TraceGlobal(JSTracer* aTrc, JSObject* aObj);
   static bool PostCreateGlobal(JSContext* aCx, JS::Handle<JSObject*> aGlobal);
diff --git a/dom/bindings/Bindings.conf b/dom/bindings/Bindings.conf
index 56d220194a..fe4a4eaeff 100644
--- a/dom/bindings/Bindings.conf
+++ b/dom/bindings/Bindings.conf
@@ -722,6 +722,21 @@ DOMInterfaces = {
     },
 },
 
+'ResizeObservation': {
+    'nativeType': 'mozilla::dom::ResizeObservation',
+    'headerFile': 'mozilla/dom/ResizeObserver.h',
+},
+
+'ResizeObserver': {
+    'nativeType': 'mozilla::dom::ResizeObserver',
+    'headerFile': 'mozilla/dom/ResizeObserver.h',
+},
+
+'ResizeObserverEntry': {
+    'nativeType': 'mozilla::dom::ResizeObserverEntry',
+    'headerFile': 'mozilla/dom/ResizeObserver.h',
+},
+
 'Response': {
     'binaryNames': { 'headers': 'headers_' },
 },
diff --git a/dom/bindings/DOMJSClass.h b/dom/bindings/DOMJSClass.h
index 6e779840fe..c97cf93221 100644
--- a/dom/bindings/DOMJSClass.h
+++ b/dom/bindings/DOMJSClass.h
@@ -210,7 +210,7 @@ struct NativePropertiesN {
 
   const int32_t iteratorAliasMethodIndex;
 
-  constexpr const NativePropertiesN<7>* Upcast() const {
+  const NativePropertiesN<7>* Upcast() const {
     return reinterpret_cast<const NativePropertiesN<7>*>(this);
   }
 
diff --git a/dom/bindings/ErrorResult.h b/dom/bindings/ErrorResult.h
index da7ec30e71..15cf2aa036 100644
--- a/dom/bindings/ErrorResult.h
+++ b/dom/bindings/ErrorResult.h
@@ -56,7 +56,7 @@ enum ErrNum {
 
 // Debug-only compile-time table of the number of arguments of each error, for use in static_assert.
 #if defined(DEBUG) && (defined(__clang__) || defined(__GNUC__))
-uint16_t constexpr ErrorFormatNumArgs[] = {
+uint16_t ErrorFormatNumArgs[] = {
 #define MSG_DEF(_name, _argc, _exn, _str) \
   _argc,
 #include "mozilla/dom/Errors.msg"
diff --git a/dom/cache/DBSchema.cpp b/dom/cache/DBSchema.cpp
index 176e7b9d1d..37510e2e15 100644
--- a/dom/cache/DBSchema.cpp
+++ b/dom/cache/DBSchema.cpp
@@ -2460,7 +2460,7 @@ Validate(mozIStorageConnection* aConn)
 typedef nsresult (*MigrationFunc)(mozIStorageConnection*, bool&);
 struct Migration
 {
-  constexpr Migration(int32_t aFromVersion, MigrationFunc aFunc)
+  Migration(int32_t aFromVersion, MigrationFunc aFunc)
     : mFromVersion(aFromVersion)
     , mFunc(aFunc)
   { }
diff --git a/dom/events/EventStates.h b/dom/events/EventStates.h
index 3397110bae..f23ae98731 100644
--- a/dom/events/EventStates.h
+++ b/dom/events/EventStates.h
@@ -27,7 +27,7 @@ public:
   typedef uint64_t InternalType;
   typedef uint8_t ServoType;
 
-  constexpr EventStates()
+  EventStates()
     : mStates(0)
   {
   }
@@ -37,12 +37,12 @@ public:
   // In that case, we could be sure that only macros at the end were creating
   // EventStates instances with mStates set to something else than 0.
   // Unfortunately, this constructor is needed at at least two places now.
-  explicit constexpr EventStates(InternalType aStates)
+  explicit EventStates(InternalType aStates)
     : mStates(aStates)
   {
   }
 
-  EventStates constexpr operator|(const EventStates& aEventStates) const
+  EventStates operator|(const EventStates& aEventStates) const
   {
     return EventStates(mStates | aEventStates.mStates);
   }
@@ -56,7 +56,7 @@ public:
   // NOTE: calling if (eventStates1 & eventStates2) will not build.
   // This might work correctly if operator bool() is defined
   // but using HasState, HasAllStates or HasAtLeastOneOfStates is recommended.
-  EventStates constexpr operator&(const EventStates& aEventStates) const
+  EventStates operator&(const EventStates& aEventStates) const
   {
     return EventStates(mStates & aEventStates.mStates);
   }
diff --git a/dom/heapsnapshot/HeapSnapshot.cpp b/dom/heapsnapshot/HeapSnapshot.cpp
index 299a96a9cc..668b7f5a5d 100644
--- a/dom/heapsnapshot/HeapSnapshot.cpp
+++ b/dom/heapsnapshot/HeapSnapshot.cpp
@@ -1455,7 +1455,7 @@ HeapSnapshot::CreateUniqueCoreDumpFile(ErrorResult& rv,
 class DeleteHeapSnapshotTempFileHelperChild
 {
 public:
-  constexpr DeleteHeapSnapshotTempFileHelperChild() { }
+  DeleteHeapSnapshotTempFileHelperChild() { }
 
   void operator()(PHeapSnapshotTempFileHelperChild* ptr) const {
     Unused << NS_WARN_IF(!HeapSnapshotTempFileHelperChild::Send__delete__(ptr));
diff --git a/dom/html/HTMLInputElement.cpp b/dom/html/HTMLInputElement.cpp
index f2c14ff2dc..a1ffbb15df 100644
--- a/dom/html/HTMLInputElement.cpp
+++ b/dom/html/HTMLInputElement.cpp
@@ -5858,8 +5858,6 @@ HTMLInputElement::ParseAttribute(int32_t aNamespaceID,
                                  const nsAString& aValue,
                                  nsAttrValue& aResult)
 {
-  // We can't make these static_asserts because kInputDefaultType and
-  // kInputTypeTable aren't constexpr.
   MOZ_ASSERT(kInputDefaultType->value == NS_FORM_INPUT_TEXT,
              "Someone forgot to update kInputDefaultType when adding a new "
              "input type.");
diff --git a/dom/html/HTMLTrackElement.cpp b/dom/html/HTMLTrackElement.cpp
index 758018f8d2..59810a4447 100644
--- a/dom/html/HTMLTrackElement.cpp
+++ b/dom/html/HTMLTrackElement.cpp
@@ -57,7 +57,7 @@ namespace mozilla {
 namespace dom {
 
 // Map html attribute string values to TextTrackKind enums.
-static constexpr nsAttrValue::EnumTable kKindTable[] = {
+static nsAttrValue::EnumTable kKindTable[] = {
   { "subtitles", static_cast<int16_t>(TextTrackKind::Subtitles) },
   { "captions", static_cast<int16_t>(TextTrackKind::Captions) },
   { "descriptions", static_cast<int16_t>(TextTrackKind::Descriptions) },
@@ -68,7 +68,7 @@ static constexpr nsAttrValue::EnumTable kKindTable[] = {
 
 // Invalid values are treated as "metadata" in ParseAttribute, but if no value
 // at all is specified, it's treated as "subtitles" in GetKind
-static constexpr const nsAttrValue::EnumTable* kKindTableInvalidValueDefault = &kKindTable[4];
+static const nsAttrValue::EnumTable* kKindTableInvalidValueDefault = &kKindTable[4];
 
 class WindowDestroyObserver final : public nsIObserver
 {
diff --git a/dom/indexedDB/ActorsParent.cpp b/dom/indexedDB/ActorsParent.cpp
index 74afef4523..c7feb8d867 100644
--- a/dom/indexedDB/ActorsParent.cpp
+++ b/dom/indexedDB/ActorsParent.cpp
@@ -270,7 +270,7 @@ const uint32_t kDEBUGTransactionThreadSleepMS = 0;
 #endif
 
 template <size_t N>
-constexpr size_t
+size_t
 LiteralStringLength(const char (&aArr)[N])
 {
   static_assert(N, "Zero-length string literal?!");
@@ -2905,11 +2905,11 @@ UpgradeKeyFunction::CopyAndUpgradeKeyBufferInternal(const uint8_t*& aSource,
   MOZ_ASSERT(aDestination);
   MOZ_ASSERT(aTagOffset <=  Key::kMaxArrayCollapse);
 
-  static constexpr uint8_t kOldNumberTag = 0x1;
-  static constexpr uint8_t kOldDateTag = 0x2;
-  static constexpr uint8_t kOldStringTag = 0x3;
-  static constexpr uint8_t kOldArrayTag = 0x4;
-  static constexpr uint8_t kOldMaxType = kOldArrayTag;
+  static uint8_t kOldNumberTag = 0x1;
+  static uint8_t kOldDateTag = 0x2;
+  static uint8_t kOldStringTag = 0x3;
+  static uint8_t kOldArrayTag = 0x4;
+  static uint8_t kOldMaxType = kOldArrayTag;
 
   if (NS_WARN_IF(aRecursionDepth > Key::kMaxRecursionDepth)) {
     IDB_REPORT_INTERNAL_ERR();
diff --git a/dom/media/MediaCache.cpp b/dom/media/MediaCache.cpp
index 37399f8511..64523afcba 100644
--- a/dom/media/MediaCache.cpp
+++ b/dom/media/MediaCache.cpp
@@ -277,7 +277,7 @@ protected:
   };
 
   struct BlockOwner {
-    constexpr BlockOwner() {}
+    BlockOwner() {}
 
     // The stream that owns this block, or null if the block is free.
     MediaCacheStream* mStream = nullptr;
diff --git a/dom/media/platforms/PDMFactory.h b/dom/media/platforms/PDMFactory.h
index 94be7e9283..cc5c3f9fa4 100644
--- a/dom/media/platforms/PDMFactory.h
+++ b/dom/media/platforms/PDMFactory.h
@@ -45,10 +45,10 @@ public:
   // This is called on the decode task queue.
   void SetCDMProxy(CDMProxy* aProxy);
 
-  static constexpr int kYUV400 = 0;
-  static constexpr int kYUV420 = 1;
-  static constexpr int kYUV422 = 2;
-  static constexpr int kYUV444 = 3;
+  static const int kYUV400 = 0;
+  static const int kYUV420 = 1;
+  static const int kYUV422 = 2;
+  static const int kYUV444 = 3;
 
 private:
   virtual ~PDMFactory();
diff --git a/dom/smil/SMILBoolType.h b/dom/smil/SMILBoolType.h
index 608a09ccf5..d0bbcf5ea2 100644
--- a/dom/smil/SMILBoolType.h
+++ b/dom/smil/SMILBoolType.h
@@ -42,7 +42,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SMILBoolType() {}
+  SMILBoolType() {}
 };
 
 } // namespace mozilla
diff --git a/dom/smil/SMILEnumType.h b/dom/smil/SMILEnumType.h
index b6cda3ff9c..070ae60bb6 100644
--- a/dom/smil/SMILEnumType.h
+++ b/dom/smil/SMILEnumType.h
@@ -43,7 +43,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SMILEnumType() {}
+  SMILEnumType() {}
 };
 
 } // namespace mozilla
diff --git a/dom/smil/SMILIntegerType.h b/dom/smil/SMILIntegerType.h
index 39560cc796..bf8030b6eb 100644
--- a/dom/smil/SMILIntegerType.h
+++ b/dom/smil/SMILIntegerType.h
@@ -38,7 +38,8 @@ public:
   }
 
 private:
-  constexpr SMILIntegerType() {}
+  // Private constructor: prevent instances beyond my singleton.
+  SMILIntegerType() {}
 };
 
 } // namespace mozilla
diff --git a/dom/smil/SMILStringType.h b/dom/smil/SMILStringType.h
index 6f160cadbb..6fb51b7d9d 100644
--- a/dom/smil/SMILStringType.h
+++ b/dom/smil/SMILStringType.h
@@ -43,7 +43,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SMILStringType() {}
+  SMILStringType() {}
 };
 
 } // namespace mozilla
diff --git a/dom/smil/nsSMILCSSValueType.h b/dom/smil/nsSMILCSSValueType.h
index 0c71605f05..0303e69e37 100644
--- a/dom/smil/nsSMILCSSValueType.h
+++ b/dom/smil/nsSMILCSSValueType.h
@@ -110,7 +110,7 @@ public:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr nsSMILCSSValueType() {}
+  nsSMILCSSValueType() {}
 };
 
 #endif // NS_SMILCSSVALUETYPE_H_
diff --git a/dom/smil/nsSMILFloatType.h b/dom/smil/nsSMILFloatType.h
index fd57e4a770..470e935d37 100644
--- a/dom/smil/nsSMILFloatType.h
+++ b/dom/smil/nsSMILFloatType.h
@@ -41,7 +41,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr nsSMILFloatType() {}
+  nsSMILFloatType() {}
 };
 
 #endif // NS_SMILFLOATTYPE_H_
diff --git a/dom/smil/nsSMILNullType.h b/dom/smil/nsSMILNullType.h
index d21610ff41..c668773ce5 100644
--- a/dom/smil/nsSMILNullType.h
+++ b/dom/smil/nsSMILNullType.h
@@ -44,7 +44,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr nsSMILNullType() {}
+  nsSMILNullType() {}
 };
 
 #endif // NS_SMILNULLTYPE_H_
diff --git a/dom/svg/SVGIntegerPairSMILType.h b/dom/svg/SVGIntegerPairSMILType.h
index 52af5d3dc6..ae62781fdb 100644
--- a/dom/svg/SVGIntegerPairSMILType.h
+++ b/dom/svg/SVGIntegerPairSMILType.h
@@ -45,7 +45,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SVGIntegerPairSMILType() {}
+  SVGIntegerPairSMILType() {}
 };
 
 } // namespace mozilla
diff --git a/dom/svg/SVGLengthListSMILType.h b/dom/svg/SVGLengthListSMILType.h
index 10321b585b..2fbb844887 100644
--- a/dom/svg/SVGLengthListSMILType.h
+++ b/dom/svg/SVGLengthListSMILType.h
@@ -93,7 +93,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SVGLengthListSMILType() {}
+  SVGLengthListSMILType() {}
 };
 
 } // namespace mozilla
diff --git a/dom/svg/SVGMotionSMILType.h b/dom/svg/SVGMotionSMILType.h
index 7529d796d7..768c90093f 100644
--- a/dom/svg/SVGMotionSMILType.h
+++ b/dom/svg/SVGMotionSMILType.h
@@ -78,7 +78,7 @@ public:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SVGMotionSMILType() {}
+  SVGMotionSMILType() {}
 };
 
 } // namespace mozilla
diff --git a/dom/svg/SVGNumberListSMILType.h b/dom/svg/SVGNumberListSMILType.h
index 5e8db8afeb..efdabdb149 100644
--- a/dom/svg/SVGNumberListSMILType.h
+++ b/dom/svg/SVGNumberListSMILType.h
@@ -47,7 +47,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SVGNumberListSMILType() {}
+  SVGNumberListSMILType() {}
 };
 
 } // namespace mozilla
diff --git a/dom/svg/SVGNumberPairSMILType.h b/dom/svg/SVGNumberPairSMILType.h
index 0f90163f58..a88a0f5acd 100644
--- a/dom/svg/SVGNumberPairSMILType.h
+++ b/dom/svg/SVGNumberPairSMILType.h
@@ -40,7 +40,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SVGNumberPairSMILType() {}
+  SVGNumberPairSMILType() {}
 };
 
 } // namespace mozilla
diff --git a/dom/svg/SVGOrientSMILType.h b/dom/svg/SVGOrientSMILType.h
index 6b30cbee0f..904b376807 100644
--- a/dom/svg/SVGOrientSMILType.h
+++ b/dom/svg/SVGOrientSMILType.h
@@ -60,7 +60,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SVGOrientSMILType() {}
+  SVGOrientSMILType() {}
 };
 
 } // namespace mozilla
diff --git a/dom/svg/SVGPathSegListSMILType.h b/dom/svg/SVGPathSegListSMILType.h
index 6856ef2759..5e4e89141e 100644
--- a/dom/svg/SVGPathSegListSMILType.h
+++ b/dom/svg/SVGPathSegListSMILType.h
@@ -51,7 +51,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SVGPathSegListSMILType() {}
+  SVGPathSegListSMILType() {}
 };
 
 } // namespace mozilla
diff --git a/dom/svg/SVGPointListSMILType.h b/dom/svg/SVGPointListSMILType.h
index 6f58bd42e5..053959553a 100644
--- a/dom/svg/SVGPointListSMILType.h
+++ b/dom/svg/SVGPointListSMILType.h
@@ -47,7 +47,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SVGPointListSMILType() {}
+  SVGPointListSMILType() {}
 };
 
 } // namespace mozilla
diff --git a/dom/svg/SVGTransformListSMILType.h b/dom/svg/SVGTransformListSMILType.h
index 526f9ba50c..6572815a15 100644
--- a/dom/svg/SVGTransformListSMILType.h
+++ b/dom/svg/SVGTransformListSMILType.h
@@ -123,7 +123,7 @@ public:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SVGTransformListSMILType() {}
+  SVGTransformListSMILType() {}
 };
 
 } // end namespace mozilla
diff --git a/dom/svg/SVGViewBoxSMILType.h b/dom/svg/SVGViewBoxSMILType.h
index f05f3928d2..54b171263b 100644
--- a/dom/svg/SVGViewBoxSMILType.h
+++ b/dom/svg/SVGViewBoxSMILType.h
@@ -40,7 +40,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SVGViewBoxSMILType() {}
+  SVGViewBoxSMILType() {}
 };
 
 } // namespace mozilla
diff --git a/dom/webidl/ResizeObserver.webidl b/dom/webidl/ResizeObserver.webidl
new file mode 100644
index 0000000000..98700f53c6
--- /dev/null
+++ b/dom/webidl/ResizeObserver.webidl
@@ -0,0 +1,39 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * The origin of this IDL file is
+ * https://wicg.github.io/ResizeObserver/
+ */
+
+[Constructor(ResizeObserverCallback callback),
+ Exposed=Window,
+ Pref="layout.css.resizeobserver.enabled"]
+interface ResizeObserver {
+    [Throws]
+    void observe(Element? target);
+    [Throws]
+    void unobserve(Element? target);
+    void disconnect();
+};
+
+callback ResizeObserverCallback = void (sequence<ResizeObserverEntry> entries, ResizeObserver observer);
+
+[Constructor(Element? target),
+ ChromeOnly,
+ Pref="layout.css.resizeobserver.enabled"]
+interface ResizeObserverEntry {
+    readonly attribute Element target;
+    readonly attribute DOMRectReadOnly? contentRect;
+};
+
+[Constructor(Element? target),
+ ChromeOnly,
+ Pref="layout.css.resizeobserver.enabled"]
+interface ResizeObservation {
+    readonly attribute Element target;
+    readonly attribute long broadcastWidth;
+    readonly attribute long broadcastHeight;
+    boolean isActive();
+};
diff --git a/dom/webidl/moz.build b/dom/webidl/moz.build
index f16ea19478..ddd0804a4e 100644
--- a/dom/webidl/moz.build
+++ b/dom/webidl/moz.build
@@ -373,6 +373,7 @@ WEBIDL_FILES = [
     'Range.webidl',
     'Rect.webidl',
     'Request.webidl',
+    'ResizeObserver.webidl',
     'Response.webidl',
     'RGBColor.webidl',
     'RTCStatsReport.webidl',
diff --git a/dom/xbl/nsXBLMaybeCompiled.h b/dom/xbl/nsXBLMaybeCompiled.h
index d4b366b0ee..ba66ec5d51 100644
--- a/dom/xbl/nsXBLMaybeCompiled.h
+++ b/dom/xbl/nsXBLMaybeCompiled.h
@@ -124,7 +124,7 @@ struct BarrierMethods<nsXBLMaybeCompiled<UncompiledT>>
 template <class T>
 struct IsHeapConstructibleType<nsXBLMaybeCompiled<T>>
 { // Yes, this is the exception to the rule. Sorry.
-  static constexpr bool value = true;
+  static const bool value = true;
 };
 
 template <class UncompiledT>
diff --git a/layout/base/nsPresShell.cpp b/layout/base/nsPresShell.cpp
index e8670ff3bf..de3dc1a256 100644
--- a/layout/base/nsPresShell.cpp
+++ b/layout/base/nsPresShell.cpp
@@ -9060,6 +9060,11 @@ PresShell::DidDoReflow(bool aInterruptible)
     docShell->NotifyReflowObservers(aInterruptible, mLastReflowStart, now);
   }
 
+  // Notify resize observers on reflow.
+  if (!mPresContext->HasPendingInterrupt()) {
+    mDocument->ScheduleResizeObserversNotification();
+  }
+
   if (sSynthMouseMove) {
     SynthesizeMouseMove(false);
   }
diff --git a/layout/inspector/inDOMUtils.cpp b/layout/inspector/inDOMUtils.cpp
index 58afc5f760..a28b35ef8f 100644
--- a/layout/inspector/inDOMUtils.cpp
+++ b/layout/inspector/inDOMUtils.cpp
@@ -842,7 +842,7 @@ PropertySupportsVariant(nsCSSPropertyID aPropertyID, uint32_t aVariant)
       case eCSSProperty_grid_row_end:
       case eCSSProperty_font_weight:
       case eCSSProperty_initial_letter:
-        supported = VARIANT_NUMBER;
+        supported = VARIANT_NUMBER | VARIANT_OPACITY;
         break;
 
       default:
@@ -905,7 +905,7 @@ inDOMUtils::CssPropertySupportsType(const nsAString& aProperty, uint32_t aType,
     break;
   case TYPE_NUMBER:
     // Include integers under "number"?
-    variant = VARIANT_NUMBER | VARIANT_INTEGER;
+    variant = VARIANT_NUMBER | VARIANT_INTEGER | VARIANT_OPACITY;
     break;
   default:
     // Unknown type
diff --git a/layout/style/nsCSSParser.cpp b/layout/style/nsCSSParser.cpp
index 556e35406a..18411be0b0 100644
--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -1300,7 +1300,7 @@ protected:
   }
   bool ParseNonNegativeNumber(nsCSSValue& aValue)
   {
-    return ParseSingleTokenNonNegativeVariant(aValue, VARIANT_NUMBER, nullptr);
+    return ParseSingleTokenNonNegativeVariant(aValue, VARIANT_NUMBER | VARIANT_OPACITY, nullptr);
   }
 
   // Helpers for some common ParseSingleTokenOneOrLargerVariant calls.
@@ -1310,7 +1310,7 @@ protected:
   }
   bool ParseOneOrLargerNumber(nsCSSValue& aValue)
   {
-    return ParseSingleTokenOneOrLargerVariant(aValue, VARIANT_NUMBER, nullptr);
+    return ParseSingleTokenOneOrLargerVariant(aValue, VARIANT_NUMBER | VARIANT_OPACITY, nullptr);
   }
 
   // http://dev.w3.org/csswg/css-values/#custom-idents
@@ -7791,6 +7791,7 @@ CSSParserImpl::ParseNonNegativeVariant(nsCSSValue& aValue,
                                VARIANT_NUMBER |
                                VARIANT_LENGTH |
                                VARIANT_PERCENT |
+                               VARIANT_OPACITY |
                                VARIANT_INTEGER)) == 0,
              "need to update code below to handle additional variants");
 
@@ -7831,6 +7832,7 @@ CSSParserImpl::ParseOneOrLargerVariant(nsCSSValue& aValue,
   // that we specifically handle.
   MOZ_ASSERT((aVariantMask & ~(VARIANT_ALL_NONNUMERIC |
                                VARIANT_NUMBER |
+                               VARIANT_OPACITY |
                                VARIANT_INTEGER)) == 0,
              "need to update code below to handle additional variants");
 
@@ -7959,9 +7961,9 @@ CSSParserImpl::ParseVariant(nsCSSValue& aValue,
       }
     }
   }
-  // Check VARIANT_NUMBER and VARIANT_INTEGER before VARIANT_LENGTH or
-  // VARIANT_ZERO_ANGLE.
-  if (((aVariantMask & VARIANT_NUMBER) != 0) &&
+  // Check VARIANT_NUMBER, number tokens for VARIANT_OPACITY, and 
+  // VARIANT_INTEGER before VARIANT_LENGTH or VARIANT_ZERO_ANGLE.
+  if (((aVariantMask & (VARIANT_NUMBER | VARIANT_OPACITY)) != 0) &&
       (eCSSToken_Number == tk->mType)) {
     aValue.SetFloatValue(tk->mNumber, eCSSUnit_Number);
     return CSSParseResult::Ok;
@@ -7971,6 +7973,7 @@ CSSParserImpl::ParseVariant(nsCSSValue& aValue,
     aValue.SetIntValue(tk->mInteger, eCSSUnit_Integer);
     return CSSParseResult::Ok;
   }
+
   if (((aVariantMask & (VARIANT_LENGTH | VARIANT_ANGLE |
                         VARIANT_FREQUENCY | VARIANT_TIME)) != 0 &&
        eCSSToken_Dimension == tk->mType) ||
@@ -7996,6 +7999,15 @@ CSSParserImpl::ParseVariant(nsCSSValue& aValue,
     aValue.SetPercentValue(tk->mNumber);
     return CSSParseResult::Ok;
   }
+  // We need to store eCSSToken_Percentage in eCSSUnit_Number in order to 
+  // serialize opacity according to spec. All percentage tokens are stored 
+  // as floats, so no type conversion is needed to make this possible. 
+  // Percentage tokens have to be evaluated later than number tokens.
+  if (((aVariantMask & VARIANT_OPACITY) != 0) &&
+      (eCSSToken_Percentage == tk->mType)) {
+    aValue.SetFloatValue(tk->mNumber, eCSSUnit_Number);
+    return CSSParseResult::Ok;
+  }
   if (mUnitlessLengthQuirk) { // NONSTANDARD: Nav interprets unitless numbers as px
     if (((aVariantMask & VARIANT_LENGTH) != 0) &&
         (eCSSToken_Number == tk->mType)) {
@@ -8474,7 +8486,7 @@ CSSParserImpl::ParseImageRect(nsCSSValue& aImage)
       break;
     }
 
-    static const int32_t VARIANT_SIDE = VARIANT_NUMBER | VARIANT_PERCENT;
+    static const int32_t VARIANT_SIDE = VARIANT_NUMBER | VARIANT_PERCENT | VARIANT_OPACITY;
     if (!ParseSingleTokenNonNegativeVariant(top, VARIANT_SIDE, nullptr) ||
         !ExpectSymbol(',', true) ||
         !ParseSingleTokenNonNegativeVariant(right, VARIANT_SIDE, nullptr) ||
@@ -10883,7 +10895,7 @@ CSSParserImpl::ParseWebkitGradientColorStop(nsCSSValueGradient* aGradient)
   if (mToken.mIdent.LowerCaseEqualsLiteral("color-stop")) {
     // Parse stop location, followed by comma.
     if (!ParseSingleTokenVariant(stop->mLocation,
-                                 VARIANT_NUMBER | VARIANT_PERCENT,
+                                 VARIANT_NUMBER | VARIANT_PERCENT | VARIANT_OPACITY,
                                  nullptr) ||
         !ExpectSymbol(',', true)) {
       SkipUntil(')'); // Skip to end of color-stop(...) expression.
@@ -16046,7 +16058,7 @@ static bool GetFunctionParseInformation(nsCSSKeyword aToken,
     {VARIANT_LBCALC, VARIANT_LBCALC, VARIANT_LBCALC},
     {VARIANT_ANGLE_OR_ZERO},
     {VARIANT_ANGLE_OR_ZERO, VARIANT_ANGLE_OR_ZERO},
-    {VARIANT_NUMBER},
+    {VARIANT_NUMBER|VARIANT_OPACITY},
     {VARIANT_LENGTH|VARIANT_NONNEGATIVE_DIMENSION},
     {VARIANT_LB|VARIANT_NONNEGATIVE_DIMENSION},
     {VARIANT_NUMBER, VARIANT_NUMBER},
@@ -17628,7 +17640,7 @@ CSSParserImpl::ParseScrollSnapPoints(nsCSSValue& aValue, nsCSSPropertyID aPropID
       nsCSSKeywords::LookupKeyword(mToken.mIdent) == eCSSKeyword_repeat) {
     nsCSSValue lengthValue;
     if (ParseNonNegativeVariant(lengthValue,
-                                VARIANT_LENGTH | VARIANT_PERCENT | VARIANT_CALC,
+                                VARIANT_LENGTH | VARIANT_PERCENT | VARIANT_OPACITY | VARIANT_CALC,
                                 nullptr) != CSSParseResult::Ok) {
       REPORT_UNEXPECTED(PEExpectedNonnegativeNP);
       SkipUntil(')');
diff --git a/layout/style/nsCSSPropList.h b/layout/style/nsCSSPropList.h
index f62aa38272..8900192454 100644
--- a/layout/style/nsCSSPropList.h
+++ b/layout/style/nsCSSPropList.h
@@ -1703,7 +1703,7 @@ CSS_PROP_SVG(
     FillOpacity,
     CSS_PROPERTY_PARSE_VALUE,
     "",
-    VARIANT_HN | VARIANT_OPENTYPE_SVG_KEYWORD,
+    VARIANT_INHERIT | VARIANT_OPACITY | VARIANT_OPENTYPE_SVG_KEYWORD,
     kContextOpacityKTable,
     offsetof(nsStyleSVG, mFillOpacity),
     eStyleAnimType_float)
@@ -1841,7 +1841,7 @@ CSS_PROP_SVGRESET(
     FloodOpacity,
     CSS_PROPERTY_PARSE_VALUE,
     "",
-    VARIANT_HN,
+    VARIANT_INHERIT | VARIANT_OPACITY,
     nullptr,
     offsetof(nsStyleSVGReset, mFloodOpacity),
     eStyleAnimType_float)
@@ -3070,7 +3070,7 @@ CSS_PROP_EFFECTS(
         CSS_PROPERTY_CAN_ANIMATE_ON_COMPOSITOR |
         CSS_PROPERTY_CREATES_STACKING_CONTEXT,
     "",
-    VARIANT_HN,
+    VARIANT_INHERIT | VARIANT_OPACITY,
     nullptr,
     offsetof(nsStyleEffects, mOpacity),
     eStyleAnimType_float)
@@ -3761,7 +3761,7 @@ CSS_PROP_SVGRESET(
     StopOpacity,
     CSS_PROPERTY_PARSE_VALUE,
     "",
-    VARIANT_HN,
+    VARIANT_INHERIT | VARIANT_OPACITY,
     nullptr,
     offsetof(nsStyleSVGReset, mStopOpacity),
     eStyleAnimType_float)
@@ -3836,7 +3836,7 @@ CSS_PROP_SVG(
     StrokeOpacity,
     CSS_PROPERTY_PARSE_VALUE,
     "",
-    VARIANT_HN | VARIANT_OPENTYPE_SVG_KEYWORD,
+    VARIANT_INHERIT | VARIANT_OPACITY | VARIANT_OPENTYPE_SVG_KEYWORD,
     kContextOpacityKTable,
     offsetof(nsStyleSVG, mStrokeOpacity),
     eStyleAnimType_float)
diff --git a/layout/style/nsCSSProps.h b/layout/style/nsCSSProps.h
index aabbac07a7..34d457c087 100644
--- a/layout/style/nsCSSProps.h
+++ b/layout/style/nsCSSProps.h
@@ -43,6 +43,7 @@
 #define VARIANT_IDENTIFIER      0x002000  // D
 #define VARIANT_IDENTIFIER_NO_INHERIT 0x004000 // like above, but excluding
 // 'inherit' and 'initial'
+#define VARIANT_OPACITY         0x008000  // Take floats and percents as input, output float.
 #define VARIANT_AUTO            0x010000  // A
 #define VARIANT_INHERIT         0x020000  // H eCSSUnit_Initial, eCSSUnit_Inherit, eCSSUnit_Unset
 #define VARIANT_NONE            0x040000  // O
diff --git a/layout/style/test/property_database.js b/layout/style/test/property_database.js
index 2d6352148a..bc43836303 100644
--- a/layout/style/test/property_database.js
+++ b/layout/style/test/property_database.js
@@ -3540,7 +3540,7 @@ var gCSSProperties = {
     inherited: false,
     type: CSS_TYPE_LONGHAND,
     initial_values: [ "1", "17", "397.376", "3e1", "3e+1", "3e0", "3e+0", "3e-0" ],
-    other_values: [ "0", "0.4", "0.0000", "-3", "3e-1" ],
+    other_values: [ "0", "0.4", "0.0000", "-3", "3e-1" "-100%", "50%" ],
     invalid_values: [ "0px", "1px" ]
   },
   "-moz-orient": {
@@ -4272,8 +4272,8 @@ var gCSSProperties = {
     domProp: "fillOpacity",
     inherited: true,
     type: CSS_TYPE_LONGHAND,
-    initial_values: [ "1", "2.8", "1.000", "context-fill-opacity", "context-stroke-opacity" ],
-    other_values: [ "0", "0.3", "-7.3" ],
+    initial_values: [ "1", "2.8", "1.000", "300%", "context-fill-opacity", "context-stroke-opacity" ],
+    other_values: [ "0", "0.3", "-7.3", "-100%", "50%" ],
     invalid_values: []
   },
   "fill-rule": {
@@ -4305,8 +4305,8 @@ var gCSSProperties = {
     domProp: "floodOpacity",
     inherited: false,
     type: CSS_TYPE_LONGHAND,
-    initial_values: [ "1", "2.8", "1.000" ],
-    other_values: [ "0", "0.3", "-7.3" ],
+    initial_values: [ "1", "2.8", "1.000", "300%" ],
+    other_values: [ "0", "0.3", "-7.3", "-100%", "50%" ],
     invalid_values: []
   },
   "image-rendering": {
@@ -4380,8 +4380,8 @@ var gCSSProperties = {
     domProp: "stopOpacity",
     inherited: false,
     type: CSS_TYPE_LONGHAND,
-    initial_values: [ "1", "2.8", "1.000" ],
-    other_values: [ "0", "0.3", "-7.3" ],
+    initial_values: [ "1", "2.8", "1.000", "300%" ],
+    other_values: [ "0", "0.3", "-7.3", "-100%", "50%" ],
     invalid_values: []
   },
   "stroke": {
@@ -4436,8 +4436,8 @@ var gCSSProperties = {
     domProp: "strokeOpacity",
     inherited: true,
     type: CSS_TYPE_LONGHAND,
-    initial_values: [ "1", "2.8", "1.000", "context-fill-opacity", "context-stroke-opacity" ],
-    other_values: [ "0", "0.3", "-7.3" ],
+    initial_values: [ "1", "2.8", "1.000", "300%", "context-fill-opacity", "context-stroke-opacity" ],
+    other_values: [ "0", "0.3", "-7.3", "-100%", "50% ],
     invalid_values: []
   },
   "stroke-width": {
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index ef19c50e02..c672bd894c 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -2652,6 +2652,9 @@ pref("layout.css.font-loading-api.enabled", true);
 // Should stray control characters be rendered visibly?
 pref("layout.css.control-characters.visible", false);
 
+// Is support for ResizeObservers enabled?
+pref("layout.css.resizeobserver.enabled", true);
+
 // pref for which side vertical scrollbars should be on
 // 0 = end-side in UI direction
 // 1 = end-side in document/content direction