Bug 1396584 - Remove support for multiple ShadowRoots

Tag #1375

27 files changed, 44 insertions, 1534 deletions

diff --git a/dom/base/ChildIterator.cpp b/dom/base/ChildIterator.cpp
index 23b919516b..391b7c326a 100644
--- a/dom/base/ChildIterator.cpp
+++ b/dom/base/ChildIterator.cpp
@@ -8,7 +8,6 @@
 #include "nsContentUtils.h"
 #include "mozilla/dom/XBLChildrenElement.h"
 #include "mozilla/dom/HTMLContentElement.h"
-#include "mozilla/dom/HTMLShadowElement.h"
 #include "mozilla/dom/ShadowRoot.h"
 #include "nsIAnonymousContentCreator.h"
 #include "nsIFrame.h"
@@ -78,17 +77,6 @@ ExplicitChildIterator::GetNextChild()
     }
     mIndexInInserted = 0;
     mChild = mChild->GetNextSibling();
-  } else if (mShadowIterator) {
-    // If we're inside of a <shadow> element, look through the
-    // explicit children of the projected ShadowRoot via
-    // the mShadowIterator.
-    nsIContent* nextChild = mShadowIterator->GetNextChild();
-    if (nextChild) {
-      return nextChild;
-    }
-
-    mShadowIterator = nullptr;
-    mChild = mChild->GetNextSibling();
   } else if (mDefaultChild) {
     // If we're already in default content, check if there are more nodes there
     MOZ_ASSERT(mChild);
@@ -110,23 +98,7 @@ ExplicitChildIterator::GetNextChild()
   // Iterate until we find a non-insertion point, or an insertion point with
   // content.
   while (mChild) {
-    // If the current child being iterated is a shadow insertion point then
-    // the iterator needs to go into the projected ShadowRoot.
-    if (ShadowRoot::IsShadowInsertionPoint(mChild)) {
-      // Look for the next child in the projected ShadowRoot for the <shadow>
-      // element.
-      HTMLShadowElement* shadowElem = HTMLShadowElement::FromContent(mChild);
-      ShadowRoot* projectedShadow = shadowElem->GetOlderShadowRoot();
-      if (projectedShadow) {
-        mShadowIterator = new ExplicitChildIterator(projectedShadow);
-        nsIContent* nextChild = mShadowIterator->GetNextChild();
-        if (nextChild) {
-          return nextChild;
-        }
-        mShadowIterator = nullptr;
-      }
-      mChild = mChild->GetNextSibling();
-    } else if (nsContentUtils::IsContentInsertionPoint(mChild)) {
+    if (nsContentUtils::IsContentInsertionPoint(mChild)) {
       // If the current child being iterated is a content insertion point
       // then the iterator needs to return the nodes distributed into
       // the content insertion point.
@@ -198,11 +170,9 @@ ExplicitChildIterator::Seek(nsIContent* aChildToFind)
       !aChildToFind->IsRootOfAnonymousSubtree()) {
     // Fast path: just point ourselves to aChildToFind, which is a
     // normal DOM child of ours.
-    MOZ_ASSERT(!ShadowRoot::IsShadowInsertionPoint(aChildToFind));
     MOZ_ASSERT(!nsContentUtils::IsContentInsertionPoint(aChildToFind));
     mChild = aChildToFind;
     mIndexInInserted = 0;
-    mShadowIterator = nullptr;
     mDefaultChild = nullptr;
     mIsFirst = false;
     return true;
@@ -223,9 +193,8 @@ ExplicitChildIterator::Get() const
   if (mIndexInInserted) {
     MatchedNodes assignedChildren = GetMatchedNodesForPoint(mChild);
     return assignedChildren[mIndexInInserted - 1];
-  } else if (mShadowIterator)  {
-    return mShadowIterator->Get();
   }
+
   return mDefaultChild ? mDefaultChild : mChild;
 }
 
@@ -241,13 +210,6 @@ ExplicitChildIterator::GetPreviousChild()
       return assignedChildren[mIndexInInserted - 1];
     }
     mChild = mChild->GetPreviousSibling();
-  } else if (mShadowIterator) {
-    nsIContent* previousChild = mShadowIterator->GetPreviousChild();
-    if (previousChild) {
-      return previousChild;
-    }
-    mShadowIterator = nullptr;
-    mChild = mChild->GetPreviousSibling();
   } else if (mDefaultChild) {
     // If we're already in default content, check if there are more nodes there
     mDefaultChild = mDefaultChild->GetPreviousSibling();
@@ -267,22 +229,7 @@ ExplicitChildIterator::GetPreviousChild()
   // Iterate until we find a non-insertion point, or an insertion point with
   // content.
   while (mChild) {
-    if (ShadowRoot::IsShadowInsertionPoint(mChild)) {
-      // If the current child being iterated is a shadow insertion point then
-      // the iterator needs to go into the projected ShadowRoot.
-      HTMLShadowElement* shadowElem = HTMLShadowElement::FromContent(mChild);
-      ShadowRoot* projectedShadow = shadowElem->GetOlderShadowRoot();
-      if (projectedShadow) {
-        // Create a ExplicitChildIterator that begins iterating from the end.
-        mShadowIterator = new ExplicitChildIterator(projectedShadow, false);
-        nsIContent* previousChild = mShadowIterator->GetPreviousChild();
-        if (previousChild) {
-          return previousChild;
-        }
-        mShadowIterator = nullptr;
-      }
-      mChild = mChild->GetPreviousSibling();
-    } else if (nsContentUtils::IsContentInsertionPoint(mChild)) {
+    if (nsContentUtils::IsContentInsertionPoint(mChild)) {
       // If the current child being iterated is a content insertion point
       // then the iterator needs to return the nodes distributed into
       // the content insertion point.
diff --git a/dom/base/ChildIterator.h b/dom/base/ChildIterator.h
index 63425149af..f78ba7ca3d 100644
--- a/dom/base/ChildIterator.h
+++ b/dom/base/ChildIterator.h
@@ -48,15 +48,11 @@ public:
   ExplicitChildIterator(const ExplicitChildIterator& aOther)
     : mParent(aOther.mParent), mChild(aOther.mChild),
       mDefaultChild(aOther.mDefaultChild),
-      mShadowIterator(aOther.mShadowIterator ?
-                      new ExplicitChildIterator(*aOther.mShadowIterator) :
-                      nullptr),
       mIndexInInserted(aOther.mIndexInInserted), mIsFirst(aOther.mIsFirst) {}
 
   ExplicitChildIterator(ExplicitChildIterator&& aOther)
     : mParent(aOther.mParent), mChild(aOther.mChild),
       mDefaultChild(aOther.mDefaultChild),
-      mShadowIterator(Move(aOther.mShadowIterator)),
       mIndexInInserted(aOther.mIndexInInserted), mIsFirst(aOther.mIsFirst) {}
 
   nsIContent* GetNextChild();
@@ -114,11 +110,6 @@ protected:
   // to null, we continue iterating at mChild's next sibling.
   nsIContent* mDefaultChild;
 
-  // If non-null, this points to an iterator of the explicit children of
-  // the ShadowRoot projected by the current shadow element that we're
-  // iterating.
-  nsAutoPtr<ExplicitChildIterator> mShadowIterator;
-
   // If not zero, we're iterating inserted children for an insertion point. This
   // is an index into mChild's inserted children array (mChild must be an
   // nsXBLChildrenElement). The index is one past the "current" child (as
diff --git a/dom/base/DocumentFragment.cpp b/dom/base/DocumentFragment.cpp
index b3d0d9962d..3e1e2c81bf 100644
--- a/dom/base/DocumentFragment.cpp
+++ b/dom/base/DocumentFragment.cpp
@@ -125,9 +125,7 @@ DocumentFragment::Constructor(const GlobalObject& aGlobal,
   return window->GetDoc()->CreateDocumentFragment();
 }
 
-NS_IMPL_CYCLE_COLLECTION_INHERITED(DocumentFragment,
-                                   FragmentOrElement,
-                                   mHost)
+NS_IMPL_CYCLE_COLLECTION_INHERITED(DocumentFragment, FragmentOrElement, mHost)
 
 // QueryInterface implementation for DocumentFragment
 NS_INTERFACE_MAP_BEGIN(DocumentFragment)
diff --git a/dom/base/DocumentFragment.h b/dom/base/DocumentFragment.h
index 33d8d4d9d5..ccc233ff13 100644
--- a/dom/base/DocumentFragment.h
+++ b/dom/base/DocumentFragment.h
@@ -43,8 +43,7 @@ public:
 
   // nsISupports
   NS_DECL_ISUPPORTS_INHERITED
-  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(DocumentFragment,
-                                           FragmentOrElement)
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(DocumentFragment, FragmentOrElement)
 
   // interface nsIDOMNode
   NS_FORWARD_NSIDOMNODE_TO_NSINODE
@@ -123,15 +122,9 @@ public:
     return nullptr;
   }
 
-  Element* GetHost() const
-  {
-    return mHost;
-  }
+  Element* GetHost() const { return mHost; }
 
-  void SetHost(Element* aHost)
-  {
-    mHost = aHost;
-  }
+  void SetHost(Element* aHost) { mHost = aHost; }
 
   static already_AddRefed<DocumentFragment>
   Constructor(const GlobalObject& aGlobal, ErrorResult& aRv);
diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index a2b78092ad..5bbfbee9b4 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -1095,6 +1095,11 @@ Element::RemoveFromIdTable()
 already_AddRefed<ShadowRoot>
 Element::CreateShadowRoot(ErrorResult& aError)
 {
+  if (GetShadowRoot()) {
+    aError.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
+    return nullptr;
+  }
+
   nsAutoScriptBlocker scriptBlocker;
 
   RefPtr<mozilla::dom::NodeInfo> nodeInfo;
@@ -1131,24 +1136,7 @@ Element::CreateShadowRoot(ErrorResult& aError)
 
   shadowRoot->SetIsComposedDocParticipant(IsInComposedDoc());
 
-  // Replace the old ShadowRoot with the new one and let the old
-  // ShadowRoot know about the younger ShadowRoot because the old
-  // ShadowRoot is projected into the younger ShadowRoot's shadow
-  // insertion point (if it exists).
-  ShadowRoot* olderShadow = GetShadowRoot();
   SetShadowRoot(shadowRoot);
-  if (olderShadow) {
-    olderShadow->SetYoungerShadow(shadowRoot);
-
-    // Unbind children of older shadow root because they
-    // are no longer in the composed tree.
-    for (nsIContent* child = olderShadow->GetFirstChild(); child;
-         child = child->GetNextSibling()) {
-      child->UnbindFromTree(true, false);
-    }
-
-    olderShadow->SetIsComposedDocParticipant(false);
-  }
 
   // xblBinding takes ownership of docInfo.
   RefPtr<nsXBLBinding> xblBinding = new nsXBLBinding(shadowRoot, protoBinding);
diff --git a/dom/base/FragmentOrElement.cpp b/dom/base/FragmentOrElement.cpp
index d7e7a78f49..592f5447d2 100644
--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -861,23 +861,10 @@ nsIContent::GetEventTargetParent(EventChainPreVisitor& aVisitor)
   // for destination insertion points where nodes have been distributed.
   nsTArray<nsIContent*>* destPoints = GetExistingDestInsertionPoints();
   if (destPoints && !destPoints->IsEmpty()) {
-    // Push destination insertion points to aVisitor.mDestInsertionPoints
-    // excluding shadow insertion points.
-    bool didPushNonShadowInsertionPoint = false;
+    // Push destination insertion points to aVisitor.mDestInsertionPoints.
     for (uint32_t i = 0; i < destPoints->Length(); i++) {
       nsIContent* point = destPoints->ElementAt(i);
-      if (!ShadowRoot::IsShadowInsertionPoint(point)) {
-        aVisitor.mDestInsertionPoints.AppendElement(point);
-        didPushNonShadowInsertionPoint = true;
-      }
-    }
-
-    // Next node in the event path is the final destination
-    // (non-shadow) insertion point that was pushed.
-    if (didPushNonShadowInsertionPoint) {
-      parent = aVisitor.mDestInsertionPoints.LastElement();
-      aVisitor.mDestInsertionPoints.SetLength(
-        aVisitor.mDestInsertionPoints.Length() - 1);
+      aVisitor.mDestInsertionPoints.AppendElement(point);
     }
   }
 
@@ -901,10 +888,7 @@ nsIContent::GetEventTargetParent(EventChainPreVisitor& aVisitor)
       aVisitor.mDestInsertionPoints.SetLength(
         aVisitor.mDestInsertionPoints.Length() - 1);
     } else {
-      // The pool host for the youngest shadow root is shadow DOM host,
-      // for older shadow roots, it is the shadow insertion point
-      // where the shadow root is projected, nullptr if none exists.
-      parent = thisShadowRoot->GetPoolHost();
+      parent = thisShadowRoot->GetHost();
     }
   }
 
@@ -2407,9 +2391,8 @@ FragmentOrElement::SetIsElementInStyleScopeFlagOnShadowTree(bool aInStyleScope)
   NS_ASSERTION(IsElement(), "calling SetIsElementInStyleScopeFlagOnShadowTree "
                             "on a non-Element is useless");
   ShadowRoot* shadowRoot = GetShadowRoot();
-  while (shadowRoot) {
+  if (shadowRoot) {
     shadowRoot->SetIsElementInStyleScopeFlagOnSubtree(aInStyleScope);
-    shadowRoot = shadowRoot->GetOlderShadowRoot();
   }
 }
 
diff --git a/dom/base/ShadowRoot.cpp b/dom/base/ShadowRoot.cpp
index 51223a8937..c9bd58d53a 100644
--- a/dom/base/ShadowRoot.cpp
+++ b/dom/base/ShadowRoot.cpp
@@ -15,7 +15,6 @@
 #include "nsIStyleSheetLinkingElement.h"
 #include "mozilla/dom/Element.h"
 #include "mozilla/dom/HTMLContentElement.h"
-#include "mozilla/dom/HTMLShadowElement.h"
 #include "nsXBLPrototypeBinding.h"
 #include "mozilla/StyleSheet.h"
 #include "mozilla/StyleSheetInlines.h"
@@ -27,10 +26,7 @@ NS_IMPL_CYCLE_COLLECTION_CLASS(ShadowRoot)
 
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(ShadowRoot,
                                                   DocumentFragment)
-  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mPoolHost)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mStyleSheetList)
-  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mOlderShadow)
-  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mYoungerShadow)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mAssociatedBinding)
   for (auto iter = tmp->mIdentifierMap.ConstIter(); !iter.Done();
        iter.Next()) {
@@ -38,18 +34,14 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(ShadowRoot,
   }
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
-NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(ShadowRoot,
-                                                DocumentFragment)
-  if (tmp->mPoolHost) {
-    tmp->mPoolHost->RemoveMutationObserver(tmp);
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(ShadowRoot)
+  if (tmp->GetHost()) {
+    tmp->GetHost()->RemoveMutationObserver(tmp);
   }
-  NS_IMPL_CYCLE_COLLECTION_UNLINK(mPoolHost)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mStyleSheetList)
-  NS_IMPL_CYCLE_COLLECTION_UNLINK(mOlderShadow)
-  NS_IMPL_CYCLE_COLLECTION_UNLINK(mYoungerShadow)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mAssociatedBinding)
   tmp->mIdentifierMap.Clear();
-NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END_INHERITED(DocumentFragment)
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION_INHERITED(ShadowRoot)
   NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIContent)
@@ -62,9 +54,10 @@ NS_IMPL_RELEASE_INHERITED(ShadowRoot, DocumentFragment)
 ShadowRoot::ShadowRoot(Element* aElement,
                        already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
                        nsXBLPrototypeBinding* aProtoBinding)
-  : DocumentFragment(aNodeInfo), mPoolHost(aElement),
-    mProtoBinding(aProtoBinding), mShadowElement(nullptr),
-    mInsertionPointChanged(false), mIsComposedDocParticipant(false)
+  : DocumentFragment(aNodeInfo)
+  , mProtoBinding(aProtoBinding)
+  , mInsertionPointChanged(false)
+  , mIsComposedDocParticipant(false)
 {
   SetHost(aElement);
 
@@ -81,23 +74,21 @@ ShadowRoot::ShadowRoot(Element* aElement,
   // Add the ShadowRoot as a mutation observer on the host to watch
   // for mutations because the insertion points in this ShadowRoot
   // may need to be updated when the host children are modified.
-  mPoolHost->AddMutationObserver(this);
+  GetHost()->AddMutationObserver(this);
 }
 
 ShadowRoot::~ShadowRoot()
 {
-  if (mPoolHost) {
+  if (GetHost()) {
     // mPoolHost may have been unlinked or a new ShadowRoot may have been
     // creating, making this one obsolete.
-    mPoolHost->RemoveMutationObserver(this);
+    GetHost()->RemoveMutationObserver(this);
   }
 
   UnsetFlags(NODE_IS_IN_SHADOW_TREE);
 
   // nsINode destructor expects mSubtreeRoot == this.
   SetSubtreeRootPointer(this);
-
-  SetHost(nullptr);
 }
 
 JSObject*
@@ -246,15 +237,6 @@ ShadowRoot::RemoveInsertionPoint(HTMLContentElement* aInsertionPoint)
 }
 
 void
-ShadowRoot::SetYoungerShadow(ShadowRoot* aYoungerShadow)
-{
-  mYoungerShadow = aYoungerShadow;
-  mYoungerShadow->mOlderShadow = this;
-
-  ChangePoolHost(mYoungerShadow->GetShadowElement());
-}
-
-void
 ShadowRoot::RemoveDestInsertionPoint(nsIContent* aInsertionPoint,
                                      nsTArray<nsIContent*>& aDestInsertionPoints)
 {
@@ -302,8 +284,7 @@ ShadowRoot::DistributeSingleNode(nsIContent* aContent)
     // Find the appropriate position in the matched node list for the
     // newly distributed content.
     bool isIndexFound = false;
-    MOZ_ASSERT(mPoolHost, "Where did the content come from if there is no pool host?");
-    ExplicitChildIterator childIterator(mPoolHost);
+    ExplicitChildIterator childIterator(GetHost());
     for (uint32_t i = 0; i < matchedNodes.Length(); i++) {
       // Seek through the host's explicit children until the inserted content
       // is found or when the current matched node is reached.
@@ -323,15 +304,6 @@ ShadowRoot::DistributeSingleNode(nsIContent* aContent)
       insertionPoint->AppendMatchedNode(aContent);
     }
 
-    // Handle the case where the parent of the insertion point is a ShadowRoot
-    // that is projected into the younger ShadowRoot's shadow insertion point.
-    // The node distributed into the insertion point must be reprojected
-    // to the shadow insertion point.
-    if (insertionPoint->GetParent() == this &&
-        mYoungerShadow && mYoungerShadow->GetShadowElement()) {
-      mYoungerShadow->GetShadowElement()->DistributeSingleNode(aContent);
-    }
-
     // Handle the case where the parent of the insertion point has a ShadowRoot.
     // The node distributed into the insertion point must be reprojected to the
     // insertion points of the parent's ShadowRoot.
@@ -339,16 +311,6 @@ ShadowRoot::DistributeSingleNode(nsIContent* aContent)
     if (parentShadow) {
       parentShadow->DistributeSingleNode(aContent);
     }
-
-    // Handle the case where the parent of the insertion point is the <shadow>
-    // element. The node distributed into the insertion point must be reprojected
-    // into the older ShadowRoot's insertion points.
-    if (mShadowElement && mShadowElement == insertionPoint->GetParent()) {
-      ShadowRoot* olderShadow = mShadowElement->GetOlderShadowRoot();
-      if (olderShadow) {
-        olderShadow->DistributeSingleNode(aContent);
-      }
-    }
   }
 }
 
@@ -370,15 +332,6 @@ ShadowRoot::RemoveDistributedNode(nsIContent* aContent)
 
       mInsertionPoints[i]->RemoveMatchedNode(aContent);
 
-      // Handle the case where the parent of the insertion point is a ShadowRoot
-      // that is projected into the younger ShadowRoot's shadow insertion point.
-      // The removed node needs to be removed from the shadow insertion point.
-      if (mInsertionPoints[i]->GetParent() == this) {
-        if (mYoungerShadow && mYoungerShadow->GetShadowElement()) {
-          mYoungerShadow->GetShadowElement()->RemoveDistributedNode(aContent);
-        }
-      }
-
       // Handle the case where the parent of the insertion point has a ShadowRoot.
       // The removed node needs to be removed from the insertion points of the
       // parent's ShadowRoot.
@@ -387,16 +340,6 @@ ShadowRoot::RemoveDistributedNode(nsIContent* aContent)
         parentShadow->RemoveDistributedNode(aContent);
       }
 
-      // Handle the case where the parent of the insertion point is the <shadow>
-      // element. The removed node must be removed from the older ShadowRoot's
-      // insertion points.
-      if (mShadowElement && mShadowElement == mInsertionPoints[i]->GetParent()) {
-        ShadowRoot* olderShadow = mShadowElement->GetOlderShadowRoot();
-        if (olderShadow) {
-          olderShadow->RemoveDistributedNode(aContent);
-        }
-      }
-
       break;
     }
   }
@@ -407,16 +350,10 @@ ShadowRoot::DistributeAllNodes()
 {
   // Create node pool.
   nsTArray<nsIContent*> nodePool;
-
-  // Make sure there is a pool host, an older shadow may not have
-  // one if the younger shadow does not have a <shadow> element.
-  if (mPoolHost) {
-    ExplicitChildIterator childIterator(mPoolHost);
-    for (nsIContent* content = childIterator.GetNextChild();
-         content;
-         content = childIterator.GetNextChild()) {
-      nodePool.AppendElement(content);
-    }
+  ExplicitChildIterator childIterator(GetHost());
+  for (nsIContent* content = childIterator.GetNextChild(); content;
+       content = childIterator.GetNextChild()) {
+    nodePool.AppendElement(content);
   }
 
   nsTArray<ShadowRoot*> shadowsToUpdate;
@@ -447,20 +384,6 @@ ShadowRoot::DistributeAllNodes()
     }
   }
 
-  // If there is a shadow insertion point in this ShadowRoot, the children
-  // of the shadow insertion point needs to be distributed into the insertion
-  // points of the older ShadowRoot.
-  if (mShadowElement && mOlderShadow) {
-    mOlderShadow->DistributeAllNodes();
-  }
-
-  // If there is a younger ShadowRoot with a shadow insertion point,
-  // then the children of this ShadowRoot needs to be distributed to
-  // the younger ShadowRoot's shadow insertion point.
-  if (mYoungerShadow && mYoungerShadow->GetShadowElement()) {
-    mYoungerShadow->GetShadowElement()->DistributeAllNodes();
-  }
-
   for (uint32_t i = 0; i < shadowsToUpdate.Length(); i++) {
     shadowsToUpdate[i]->DistributeAllNodes();
   }
@@ -517,59 +440,6 @@ ShadowRoot::StyleSheets()
   return mStyleSheetList;
 }
 
-void
-ShadowRoot::SetShadowElement(HTMLShadowElement* aShadowElement)
-{
-  // If there is already a shadow element point, remove
-  // the projected shadow because it is no longer an insertion
-  // point.
-  if (mShadowElement) {
-    mShadowElement->SetProjectedShadow(nullptr);
-  }
-
-  if (mOlderShadow) {
-    // Nodes for distribution will come from the new shadow element.
-    mOlderShadow->ChangePoolHost(aShadowElement);
-  }
-
-  // Set the new shadow element to project the older ShadowRoot because
-  // it is the current shadow insertion point.
-  mShadowElement = aShadowElement;
-  if (mShadowElement) {
-    mShadowElement->SetProjectedShadow(mOlderShadow);
-  }
-}
-
-void
-ShadowRoot::ChangePoolHost(nsIContent* aNewHost)
-{
-  if (mPoolHost) {
-    mPoolHost->RemoveMutationObserver(this);
-  }
-
-  // Clear the nodes matched to content insertion points
-  // because it is no longer relevant.
-  for (uint32_t i = 0; i < mInsertionPoints.Length(); i++) {
-    mInsertionPoints[i]->ClearMatchedNodes();
-  }
-
-  mPoolHost = aNewHost;
-  if (mPoolHost) {
-    mPoolHost->AddMutationObserver(this);
-  }
-}
-
-bool
-ShadowRoot::IsShadowInsertionPoint(nsIContent* aContent)
-{
-  if (!aContent) {
-    return false;
-  }
-
-  HTMLShadowElement* shadowElem = HTMLShadowElement::FromContent(aContent);
-  return shadowElem && shadowElem->IsInsertionPoint();
-}
-
 /**
  * Returns whether the web components pool population algorithm
  * on the host would contain |aContent|. This function ignores
@@ -580,8 +450,7 @@ bool
 ShadowRoot::IsPooledNode(nsIContent* aContent, nsIContent* aContainer,
                          nsIContent* aHost)
 {
-  if (nsContentUtils::IsContentInsertionPoint(aContent) ||
-      IsShadowInsertionPoint(aContent)) {
+  if (nsContentUtils::IsContentInsertionPoint(aContent)) {
     // Insertion points never end up in the pool.
     return false;
   }
@@ -614,7 +483,7 @@ ShadowRoot::AttributeChanged(nsIDocument* aDocument,
                              int32_t aModType,
                              const nsAttrValue* aOldValue)
 {
-  if (!IsPooledNode(aElement, aElement->GetParent(), mPoolHost)) {
+  if (!IsPooledNode(aElement, aElement->GetParent(), GetHost())) {
     return;
   }
 
@@ -647,7 +516,7 @@ ShadowRoot::ContentAppended(nsIDocument* aDocument,
       }
     }
 
-    if (IsPooledNode(currentChild, aContainer, mPoolHost)) {
+    if (IsPooledNode(currentChild, aContainer, GetHost())) {
       DistributeSingleNode(currentChild);
     }
 
@@ -669,7 +538,7 @@ ShadowRoot::ContentInserted(nsIDocument* aDocument,
 
   // Watch for new nodes added to the pool because the node
   // may need to be added to an insertion point.
-  if (IsPooledNode(aChild, aContainer, mPoolHost)) {
+  if (IsPooledNode(aChild, aContainer, GetHost())) {
     // Add insertion point to destination insertion points of fallback content.
     if (nsContentUtils::IsContentInsertionPoint(aContainer)) {
       HTMLContentElement* content = HTMLContentElement::FromContent(aContainer);
@@ -706,7 +575,7 @@ ShadowRoot::ContentRemoved(nsIDocument* aDocument,
 
   // Watch for node that is removed from the pool because
   // it may need to be removed from an insertion point.
-  if (IsPooledNode(aChild, aContainer, mPoolHost)) {
+  if (IsPooledNode(aChild, aContainer, GetHost())) {
     RemoveDistributedNode(aChild);
   }
 }
@@ -718,15 +587,6 @@ ShadowRoot::Clone(mozilla::dom::NodeInfo *aNodeInfo, nsINode **aResult) const
   return NS_ERROR_DOM_DATA_CLONE_ERR;
 }
 
-void
-ShadowRoot::DestroyContent()
-{
-  if (mOlderShadow) {
-    mOlderShadow->DestroyContent();
-  }
-  DocumentFragment::DestroyContent();
-}
-
 NS_IMPL_CYCLE_COLLECTION_INHERITED(ShadowRootStyleSheetList, StyleSheetList,
                                    mShadowRoot)
 
diff --git a/dom/base/ShadowRoot.h b/dom/base/ShadowRoot.h
index e76e4ab3f3..04943752be 100644
--- a/dom/base/ShadowRoot.h
+++ b/dom/base/ShadowRoot.h
@@ -25,7 +25,6 @@ namespace dom {
 
 class Element;
 class HTMLContentElement;
-class HTMLShadowElement;
 class ShadowRootStyleSheetList;
 
 class ShadowRoot final : public DocumentFragment,
@@ -42,7 +41,8 @@ public:
   NS_DECL_NSIMUTATIONOBSERVER_CONTENTINSERTED
   NS_DECL_NSIMUTATIONOBSERVER_CONTENTREMOVED
 
-  ShadowRoot(Element* aElement, already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
+  ShadowRoot(Element* aElement,
+             already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
              nsXBLPrototypeBinding* aProtoBinding);
 
   void AddToIdTable(Element* aElement, nsIAtom* aId);
@@ -52,24 +52,6 @@ public:
   bool ApplyAuthorStyles();
   void SetApplyAuthorStyles(bool aApplyAuthorStyles);
   StyleSheetList* StyleSheets();
-  HTMLShadowElement* GetShadowElement() { return mShadowElement; }
-
-  /**
-   * Sets the current shadow insertion point where the older
-   * ShadowRoot will be projected.
-   */
-  void SetShadowElement(HTMLShadowElement* aShadowElement);
-
-  /**
-   * Change the node that populates the distribution pool with
-   * its children. This is distinct from the ShadowRoot host described
-   * in the specifications. The ShadowRoot host is the element
-   * which created this ShadowRoot and does not change. The pool host
-   * is the same as the ShadowRoot host if this is the youngest
-   * ShadowRoot. If this is an older ShadowRoot, the pool host is
-   * the <shadow> element in the younger ShadowRoot (if it exists).
-   */
-  void ChangePoolHost(nsIContent* aNewHost);
 
   /**
    * Distributes a single explicit child of the pool host to the content
@@ -92,23 +74,15 @@ public:
   void AddInsertionPoint(HTMLContentElement* aInsertionPoint);
   void RemoveInsertionPoint(HTMLContentElement* aInsertionPoint);
 
-  void SetYoungerShadow(ShadowRoot* aYoungerShadow);
-  ShadowRoot* GetYoungerShadowRoot() { return mYoungerShadow; }
   void SetInsertionPointChanged() { mInsertionPointChanged = true; }
 
   void SetAssociatedBinding(nsXBLBinding* aBinding) { mAssociatedBinding = aBinding; }
 
-  nsISupports* GetParentObject() const { return mPoolHost; }
-
-  nsIContent* GetPoolHost() { return mPoolHost; }
-  nsTArray<HTMLShadowElement*>& ShadowDescendants() { return mShadowDescendants; }
-
   JSObject* WrapObject(JSContext* aCx, JS::Handle<JSObject*> aGivenProto) override;
 
   static bool IsPooledNode(nsIContent* aChild, nsIContent* aContainer,
                            nsIContent* aHost);
   static ShadowRoot* FromNode(nsINode* aNode);
-  static bool IsShadowInsertionPoint(nsIContent* aContent);
 
   static void RemoveDestInsertionPoint(nsIContent* aInsertionPoint,
                                        nsTArray<nsIContent*>& aDestInsertionPoints);
@@ -125,7 +99,6 @@ public:
   void GetInnerHTML(nsAString& aInnerHTML);
   void SetInnerHTML(const nsAString& aInnerHTML, ErrorResult& aError);
   Element* Host();
-  ShadowRoot* GetOlderShadowRoot() { return mOlderShadow; }
   void StyleSheetChanged();
 
   bool IsComposedDocParticipant() { return mIsComposedDocParticipant; }
@@ -134,14 +107,9 @@ public:
     mIsComposedDocParticipant = aIsComposedDocParticipant;
   }
 
-  virtual void DestroyContent() override;
 protected:
   virtual ~ShadowRoot();
 
-  // The pool host is the parent of the nodes that will be distributed
-  // into the insertion points in this ShadowRoot. See |ChangeShadowRoot|.
-  nsCOMPtr<nsIContent> mPoolHost;
-
   // An array of content insertion points that are a descendant of the ShadowRoot
   // sorted in tree order. Insertion points are responsible for notifying
   // the ShadowRoot when they are removed or added as a descendant. The insertion
@@ -149,10 +117,6 @@ protected:
   // by the array.
   nsTArray<HTMLContentElement*> mInsertionPoints;
 
-  // An array of the <shadow> elements that are descendant of the ShadowRoot
-  // sorted in tree order. Only the first may be a shadow insertion point.
-  nsTArray<HTMLShadowElement*> mShadowDescendants;
-
   nsTHashtable<nsIdentifierMapEntry> mIdentifierMap;
   nsXBLPrototypeBinding* mProtoBinding;
 
@@ -163,17 +127,6 @@ protected:
 
   RefPtr<ShadowRootStyleSheetList> mStyleSheetList;
 
-  // The current shadow insertion point of this ShadowRoot.
-  HTMLShadowElement* mShadowElement;
-
-  // The ShadowRoot that was created by the host element before
-  // this ShadowRoot was created.
-  RefPtr<ShadowRoot> mOlderShadow;
-
-  // The ShadowRoot that was created by the host element after
-  // this ShadowRoot was created.
-  RefPtr<ShadowRoot> mYoungerShadow;
-
   // A boolean that indicates that an insertion point was added or removed
   // from this ShadowRoot and that the nodes need to be redistributed into
   // the insertion points. After this flag is set, nodes will be distributed
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index a68f5ef87d..96f9065803 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -46,7 +46,6 @@
 #include "mozilla/dom/HTMLMediaElement.h"
 #include "mozilla/dom/HTMLTemplateElement.h"
 #include "mozilla/dom/HTMLContentElement.h"
-#include "mozilla/dom/HTMLShadowElement.h"
 #include "mozilla/dom/ipc/BlobChild.h"
 #include "mozilla/dom/ipc/BlobParent.h"
 #include "mozilla/dom/Promise.h"
@@ -7056,20 +7055,6 @@ nsContentUtils::HasDistributedChildren(nsIContent* aContent)
     return true;
   }
 
-  ShadowRoot* shadow = ShadowRoot::FromNode(aContent);
-  if (shadow) {
-    // Children of a shadow root are distributed to
-    // the shadow insertion point of the younger shadow root.
-    return shadow->GetYoungerShadowRoot();
-  }
-
-  HTMLShadowElement* shadowEl = HTMLShadowElement::FromContent(aContent);
-  if (shadowEl && shadowEl->IsInsertionPoint()) {
-    // Children of a shadow insertion points are distributed
-    // to the insertion points in the older shadow root.
-    return shadowEl->GetOlderShadowRoot();
-  }
-
   HTMLContentElement* contentEl = HTMLContentElement::FromContent(aContent);
   if (contentEl && contentEl->IsInsertionPoint()) {
     // Children of a content insertion point are distributed to the
diff --git a/dom/base/nsGkAtomList.h b/dom/base/nsGkAtomList.h
index 2f0e30387c..0ae35480ff 100644
--- a/dom/base/nsGkAtomList.h
+++ b/dom/base/nsGkAtomList.h
@@ -1560,7 +1560,6 @@ GK_ATOM(saturate, "saturate")
 GK_ATOM(saturation, "saturation")
 GK_ATOM(set, "set")
 GK_ATOM(seed, "seed")
-GK_ATOM(shadow, "shadow")
 GK_ATOM(shape_rendering, "shape-rendering")
 GK_ATOM(skewX, "skewX")
 GK_ATOM(skewY, "skewY")
diff --git a/dom/base/nsNodeUtils.cpp b/dom/base/nsNodeUtils.cpp
index 384e56cde6..5fdd24e439 100644
--- a/dom/base/nsNodeUtils.cpp
+++ b/dom/base/nsNodeUtils.cpp
@@ -65,7 +65,7 @@ using mozilla::AutoJSContext;
     }                                                             \
     ShadowRoot* shadow = ShadowRoot::FromNode(node);              \
     if (shadow) {                                                 \
-      node = shadow->GetPoolHost();                               \
+      node = shadow->GetHost();                                   \
     } else {                                                      \
       node = node->GetParentNode();                               \
     }                                                             \
@@ -93,7 +93,7 @@ using mozilla::AutoJSContext;
     }                                                             \
     ShadowRoot* shadow = ShadowRoot::FromNode(node);              \
     if (shadow) {                                                 \
-      node = shadow->GetPoolHost();                               \
+      node = shadow->GetHost();                                   \
     } else {                                                      \
       node = node->GetParentNode();                               \
     }                                                             \
diff --git a/dom/base/test/test_mutationobservers.html b/dom/base/test/test_mutationobservers.html
index 7e4c994235..021bedf1c8 100644
--- a/dom/base/test/test_mutationobservers.html
+++ b/dom/base/test/test_mutationobservers.html
@@ -612,7 +612,7 @@ function testOutsideShadowDOM() {
     is(records.length, 1);
     is(records[0].type, "attributes", "Should have got attributes");
     observer.disconnect();
-    then(testInsideShadowDOM);
+    then(testMarquee);
   });
   m.observe(div, {
       attributes: true,
@@ -628,32 +628,6 @@ function testOutsideShadowDOM() {
   div.setAttribute("foo", "bar");
 }
 
-function testInsideShadowDOM() {
-  var m = new M(function(records, observer) {
-    is(records.length, 4);
-    is(records[0].type, "childList");
-    is(records[1].type, "attributes");
-    is(records[2].type, "characterData");
-    is(records[3].type, "childList");
-    observer.disconnect();
-    then(testMarquee);
-  });
-  var sr = div.createShadowRoot();
-  m.observe(sr, {
-      attributes: true,
-      childList: true,
-      characterData: true,
-      subtree: true
-    });
-
-  sr.innerHTML = "<div" + ">text</" + "div>";
-  sr.firstChild.setAttribute("foo", "bar");
-  sr.firstChild.firstChild.data = "text2";
-  sr.firstChild.appendChild(document.createElement("div"));
-  div.setAttribute("foo", "bar2");
-
-}
-
 function testMarquee() {
   var m = new M(function(records, observer) {
     is(records.length, 1);
diff --git a/dom/html/HTMLShadowElement.cpp b/dom/html/HTMLShadowElement.cpp
deleted file mode 100644
index 8824c2875f..0000000000
--- a/dom/html/HTMLShadowElement.cpp
+++ /dev/null
@@ -1,373 +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/. */
-
-#include "mozilla/dom/ShadowRoot.h"
-
-#include "ChildIterator.h"
-#include "nsContentUtils.h"
-#include "nsDocument.h"
-#include "mozilla/dom/HTMLShadowElement.h"
-#include "mozilla/dom/HTMLUnknownElement.h"
-#include "mozilla/dom/HTMLShadowElementBinding.h"
-
-// Expand NS_IMPL_NS_NEW_HTML_ELEMENT(Shadow) to add check for web components
-// being enabled.
-nsGenericHTMLElement*
-NS_NewHTMLShadowElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
-                        mozilla::dom::FromParser aFromParser)
-{
-  // When this check is removed, remove the nsDocument.h and
-  // HTMLUnknownElement.h includes.  Also remove nsINode::IsHTMLShadowElement.
-  //
-  // We have to jump through some hoops to be able to produce both NodeInfo* and
-  // already_AddRefed<NodeInfo>& for our callees.
-  RefPtr<mozilla::dom::NodeInfo> nodeInfo(aNodeInfo);
-  if (!nsDocument::IsWebComponentsEnabled(nodeInfo)) {
-    already_AddRefed<mozilla::dom::NodeInfo> nodeInfoArg(nodeInfo.forget());
-    return new mozilla::dom::HTMLUnknownElement(nodeInfoArg);
-  }
-
-  already_AddRefed<mozilla::dom::NodeInfo> nodeInfoArg(nodeInfo.forget());
-  return new mozilla::dom::HTMLShadowElement(nodeInfoArg);
-}
-
-using namespace mozilla::dom;
-
-HTMLShadowElement::HTMLShadowElement(already_AddRefed<mozilla::dom::NodeInfo>& aNodeInfo)
-  : nsGenericHTMLElement(aNodeInfo), mIsInsertionPoint(false)
-{
-}
-
-HTMLShadowElement::~HTMLShadowElement()
-{
-  if (mProjectedShadow) {
-    mProjectedShadow->RemoveMutationObserver(this);
-  }
-}
-
-NS_IMPL_CYCLE_COLLECTION_CLASS(HTMLShadowElement)
-
-NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(HTMLShadowElement,
-                                                  nsGenericHTMLElement)
-  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mProjectedShadow)
-NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
-
-NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(HTMLShadowElement,
-                                                nsGenericHTMLElement)
-  if (tmp->mProjectedShadow) {
-    tmp->mProjectedShadow->RemoveMutationObserver(tmp);
-    tmp->mProjectedShadow = nullptr;
-  }
-NS_IMPL_CYCLE_COLLECTION_UNLINK_END
-
-NS_IMPL_ADDREF_INHERITED(HTMLShadowElement, Element)
-NS_IMPL_RELEASE_INHERITED(HTMLShadowElement, Element)
-
-NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION_INHERITED(HTMLShadowElement)
-NS_INTERFACE_MAP_END_INHERITING(nsGenericHTMLElement)
-
-NS_IMPL_ELEMENT_CLONE(HTMLShadowElement)
-
-JSObject*
-HTMLShadowElement::WrapNode(JSContext *aCx, JS::Handle<JSObject*> aGivenProto)
-{
-  return HTMLShadowElementBinding::Wrap(aCx, this, aGivenProto);
-}
-
-void
-HTMLShadowElement::SetProjectedShadow(ShadowRoot* aProjectedShadow)
-{
-  if (mProjectedShadow) {
-    mProjectedShadow->RemoveMutationObserver(this);
-
-    // The currently projected ShadowRoot is going away,
-    // thus the destination insertion points need to be updated.
-    ExplicitChildIterator childIterator(mProjectedShadow);
-    for (nsIContent* content = childIterator.GetNextChild();
-         content;
-         content = childIterator.GetNextChild()) {
-      ShadowRoot::RemoveDestInsertionPoint(this, content->DestInsertionPoints());
-    }
-  }
-
-  mProjectedShadow = aProjectedShadow;
-  if (mProjectedShadow) {
-    // A new ShadowRoot is being projected, thus its explcit
-    // children will be distributed to this shadow insertion point.
-    ExplicitChildIterator childIterator(mProjectedShadow);
-    for (nsIContent* content = childIterator.GetNextChild();
-         content;
-         content = childIterator.GetNextChild()) {
-      content->DestInsertionPoints().AppendElement(this);
-    }
-
-    // Watch for mutations on the projected shadow because
-    // it affects the nodes that are distributed to this shadow
-    // insertion point.
-    mProjectedShadow->AddMutationObserver(this);
-  }
-}
-
-static bool
-IsInFallbackContent(nsIContent* aContent)
-{
-  nsINode* parentNode = aContent->GetParentNode();
-  while (parentNode) {
-    if (parentNode->IsHTMLElement(nsGkAtoms::content)) {
-      return true;
-    }
-    parentNode = parentNode->GetParentNode();
-  }
-
-  return false;
-}
-
-nsresult
-HTMLShadowElement::BindToTree(nsIDocument* aDocument,
-                              nsIContent* aParent,
-                              nsIContent* aBindingParent,
-                              bool aCompileEventHandlers)
-{
-  RefPtr<ShadowRoot> oldContainingShadow = GetContainingShadow();
-
-  nsresult rv = nsGenericHTMLElement::BindToTree(aDocument, aParent,
-                                                 aBindingParent,
-                                                 aCompileEventHandlers);
-  NS_ENSURE_SUCCESS(rv, rv);
-
-  ShadowRoot* containingShadow = GetContainingShadow();
-  if (containingShadow && !oldContainingShadow) {
-    // Keep track of all descendant <shadow> elements in tree order so
-    // that when the current shadow insertion point is removed, the next
-    // one can be found quickly.
-    TreeOrderComparator comparator;
-    containingShadow->ShadowDescendants().InsertElementSorted(this, comparator);
-
-    if (containingShadow->ShadowDescendants()[0] != this) {
-      // Only the first <shadow> (in tree order) of a ShadowRoot can be an insertion point.
-      return NS_OK;
-    }
-
-    if (IsInFallbackContent(this)) {
-      // If the first shadow element in tree order is invalid (in fallback content),
-      // the containing ShadowRoot will not have a shadow insertion point.
-      containingShadow->SetShadowElement(nullptr);
-    } else {
-      mIsInsertionPoint = true;
-      containingShadow->SetShadowElement(this);
-    }
-
-    containingShadow->SetInsertionPointChanged();
-  }
-
-  if (mIsInsertionPoint && containingShadow) {
-    // Propagate BindToTree calls to projected shadow root children.
-    ShadowRoot* projectedShadow = containingShadow->GetOlderShadowRoot();
-    if (projectedShadow) {
-      projectedShadow->SetIsComposedDocParticipant(IsInComposedDoc());
-
-      for (nsIContent* child = projectedShadow->GetFirstChild(); child;
-           child = child->GetNextSibling()) {
-        rv = child->BindToTree(nullptr, projectedShadow,
-                               projectedShadow->GetBindingParent(),
-                               aCompileEventHandlers);
-        NS_ENSURE_SUCCESS(rv, rv);
-      }
-    }
-  }
-
-  return NS_OK;
-}
-
-void
-HTMLShadowElement::UnbindFromTree(bool aDeep, bool aNullParent)
-{
-  RefPtr<ShadowRoot> oldContainingShadow = GetContainingShadow();
-
-  if (mIsInsertionPoint && oldContainingShadow) {
-    // Propagate UnbindFromTree call to previous projected shadow
-    // root children.
-    ShadowRoot* projectedShadow = oldContainingShadow->GetOlderShadowRoot();
-    if (projectedShadow) {
-      for (nsIContent* child = projectedShadow->GetFirstChild(); child;
-           child = child->GetNextSibling()) {
-        child->UnbindFromTree(true, false);
-      }
-
-      projectedShadow->SetIsComposedDocParticipant(false);
-    }
-  }
-
-  nsGenericHTMLElement::UnbindFromTree(aDeep, aNullParent);
-
-  if (oldContainingShadow && !GetContainingShadow() && mIsInsertionPoint) {
-    nsTArray<HTMLShadowElement*>& shadowDescendants =
-      oldContainingShadow->ShadowDescendants();
-    shadowDescendants.RemoveElement(this);
-    oldContainingShadow->SetShadowElement(nullptr);
-
-    // Find the next shadow insertion point.
-    if (shadowDescendants.Length() > 0 &&
-        !IsInFallbackContent(shadowDescendants[0])) {
-      oldContainingShadow->SetShadowElement(shadowDescendants[0]);
-    }
-
-    oldContainingShadow->SetInsertionPointChanged();
-
-    mIsInsertionPoint = false;
-  }
-}
-
-void
-HTMLShadowElement::DistributeSingleNode(nsIContent* aContent)
-{
-  if (aContent->DestInsertionPoints().Contains(this)) {
-    // Node has already been distrbuted this this node,
-    // we are done.
-    return;
-  }
-
-  aContent->DestInsertionPoints().AppendElement(this);
-
-  // Handle the case where the shadow element is a child of
-  // a node with a ShadowRoot. The nodes that have been distributed to
-  // this shadow insertion point will need to be reprojected into the
-  // insertion points of the parent's ShadowRoot.
-  ShadowRoot* parentShadowRoot = GetParent()->GetShadowRoot();
-  if (parentShadowRoot) {
-    parentShadowRoot->DistributeSingleNode(aContent);
-    return;
-  }
-
-  // Handle the case where the parent of this shadow element is a ShadowRoot
-  // that is projected into a shadow insertion point in the younger ShadowRoot.
-  ShadowRoot* containingShadow = GetContainingShadow();
-  ShadowRoot* youngerShadow = containingShadow->GetYoungerShadowRoot();
-  if (youngerShadow && GetParent() == containingShadow) {
-    HTMLShadowElement* youngerShadowElement = youngerShadow->GetShadowElement();
-    if (youngerShadowElement) {
-      youngerShadowElement->DistributeSingleNode(aContent);
-    }
-  }
-}
-
-void
-HTMLShadowElement::RemoveDistributedNode(nsIContent* aContent)
-{
-  ShadowRoot::RemoveDestInsertionPoint(this, aContent->DestInsertionPoints());
-
-  // Handle the case where the shadow element is a child of
-  // a node with a ShadowRoot. The nodes that have been distributed to
-  // this shadow insertion point will need to be removed from the
-  // insertion points of the parent's ShadowRoot.
-  ShadowRoot* parentShadowRoot = GetParent()->GetShadowRoot();
-  if (parentShadowRoot) {
-    parentShadowRoot->RemoveDistributedNode(aContent);
-    return;
-  }
-
-  // Handle the case where the parent of this shadow element is a ShadowRoot
-  // that is projected into a shadow insertion point in the younger ShadowRoot.
-  ShadowRoot* containingShadow = GetContainingShadow();
-  ShadowRoot* youngerShadow = containingShadow->GetYoungerShadowRoot();
-  if (youngerShadow && GetParent() == containingShadow) {
-    HTMLShadowElement* youngerShadowElement = youngerShadow->GetShadowElement();
-    if (youngerShadowElement) {
-      youngerShadowElement->RemoveDistributedNode(aContent);
-    }
-  }
-}
-
-void
-HTMLShadowElement::DistributeAllNodes()
-{
-  // All the explicit children of the projected ShadowRoot are distributed
-  // into this shadow insertion point so update the destination insertion
-  // points.
-  ShadowRoot* containingShadow = GetContainingShadow();
-  ShadowRoot* olderShadow = containingShadow->GetOlderShadowRoot();
-  if (olderShadow) {
-    ExplicitChildIterator childIterator(olderShadow);
-    for (nsIContent* content = childIterator.GetNextChild();
-         content;
-         content = childIterator.GetNextChild()) {
-      ShadowRoot::RemoveDestInsertionPoint(this, content->DestInsertionPoints());
-      content->DestInsertionPoints().AppendElement(this);
-    }
-  }
-
-  // Handle the case where the shadow element is a child of
-  // a node with a ShadowRoot. The nodes that have been distributed to
-  // this shadow insertion point will need to be reprojected into the
-  // insertion points of the parent's ShadowRoot.
-  ShadowRoot* parentShadowRoot = GetParent()->GetShadowRoot();
-  if (parentShadowRoot) {
-    parentShadowRoot->DistributeAllNodes();
-    return;
-  }
-
-  // Handle the case where the parent of this shadow element is a ShadowRoot
-  // that is projected into a shadow insertion point in the younger ShadowRoot.
-  ShadowRoot* youngerShadow = containingShadow->GetYoungerShadowRoot();
-  if (youngerShadow && GetParent() == containingShadow) {
-    HTMLShadowElement* youngerShadowElement = youngerShadow->GetShadowElement();
-    if (youngerShadowElement) {
-      youngerShadowElement->DistributeAllNodes();
-    }
-  }
-}
-
-void
-HTMLShadowElement::ContentAppended(nsIDocument* aDocument,
-                                   nsIContent* aContainer,
-                                   nsIContent* aFirstNewContent,
-                                   int32_t aNewIndexInContainer)
-{
-  // Watch for content appended to the projected shadow (the ShadowRoot that
-  // will be rendered in place of this shadow insertion point) because the
-  // nodes may need to be distributed into other insertion points.
-  nsIContent* currentChild = aFirstNewContent;
-  while (currentChild) {
-    if (ShadowRoot::IsPooledNode(currentChild, aContainer, mProjectedShadow)) {
-      DistributeSingleNode(currentChild);
-    }
-    currentChild = currentChild->GetNextSibling();
-  }
-}
-
-void
-HTMLShadowElement::ContentInserted(nsIDocument* aDocument,
-                                   nsIContent* aContainer,
-                                   nsIContent* aChild,
-                                   int32_t aIndexInContainer)
-{
-  // Watch for content appended to the projected shadow (the ShadowRoot that
-  // will be rendered in place of this shadow insertion point) because the
-  // nodes may need to be distributed into other insertion points.
-  if (!ShadowRoot::IsPooledNode(aChild, aContainer, mProjectedShadow)) {
-    return;
-  }
-
-  DistributeSingleNode(aChild);
-}
-
-void
-HTMLShadowElement::ContentRemoved(nsIDocument* aDocument,
-                                  nsIContent* aContainer,
-                                  nsIContent* aChild,
-                                  int32_t aIndexInContainer,
-                                  nsIContent* aPreviousSibling)
-{
-  // Watch for content removed from the projected shadow (the ShadowRoot that
-  // will be rendered in place of this shadow insertion point) because the
-  // nodes may need to be removed from other insertion points.
-  if (!ShadowRoot::IsPooledNode(aChild, aContainer, mProjectedShadow)) {
-    return;
-  }
-
-  RemoveDistributedNode(aChild);
-}
-
diff --git a/dom/html/HTMLShadowElement.h b/dom/html/HTMLShadowElement.h
deleted file mode 100644
index 95083b802f..0000000000
--- a/dom/html/HTMLShadowElement.h
+++ /dev/null
@@ -1,96 +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/. */
-
-#ifndef mozilla_dom_HTMLShadowElement_h__
-#define mozilla_dom_HTMLShadowElement_h__
-
-#include "nsGenericHTMLElement.h"
-
-namespace mozilla {
-namespace dom {
-
-class HTMLShadowElement final : public nsGenericHTMLElement,
-                                public nsStubMutationObserver
-{
-public:
-  explicit HTMLShadowElement(already_AddRefed<mozilla::dom::NodeInfo>& aNodeInfo);
-
-  // nsISupports
-  NS_DECL_ISUPPORTS_INHERITED
-
-  NS_DECL_NSIMUTATIONOBSERVER_CONTENTAPPENDED
-  NS_DECL_NSIMUTATIONOBSERVER_CONTENTINSERTED
-  NS_DECL_NSIMUTATIONOBSERVER_CONTENTREMOVED
-
-  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(HTMLShadowElement,
-                                           nsGenericHTMLElement)
-
-  static HTMLShadowElement* FromContent(nsIContent* aContent)
-  {
-    if (aContent->IsHTMLShadowElement()) {
-      return static_cast<HTMLShadowElement*>(aContent);
-    }
-
-    return nullptr;
-  }
-
-  virtual bool IsHTMLShadowElement() const override { return true; }
-
-  virtual nsresult Clone(mozilla::dom::NodeInfo *aNodeInfo, nsINode **aResult) const override;
-
-  virtual nsresult BindToTree(nsIDocument* aDocument, nsIContent* aParent,
-                              nsIContent* aBindingParent,
-                              bool aCompileEventHandlers) override;
-
-  virtual void UnbindFromTree(bool aDeep = true,
-                              bool aNullParent = true) override;
-
-  bool IsInsertionPoint() { return mIsInsertionPoint; }
-
-  /**
-   * Sets the ShadowRoot that will be rendered in place of
-   * this shadow insertion point.
-   */
-  void SetProjectedShadow(ShadowRoot* aProjectedShadow);
-
-  /**
-   * Distributes a single explicit child of the projected ShadowRoot
-   * to relevant insertion points.
-   */
-  void DistributeSingleNode(nsIContent* aContent);
-
-  /**
-   * Removes a single explicit child of the projected ShadowRoot
-   * from relevant insertion points.
-   */
-  void RemoveDistributedNode(nsIContent* aContent);
-
-  /**
-   * Distributes all the explicit children of the projected ShadowRoot
-   * to the shadow insertion point in the younger ShadowRoot and
-   * the content insertion point of the parent node's ShadowRoot.
-   */
-  void DistributeAllNodes();
-
-  // WebIDL methods.
-  ShadowRoot* GetOlderShadowRoot() { return mProjectedShadow; }
-
-protected:
-  virtual ~HTMLShadowElement();
-
-  virtual JSObject* WrapNode(JSContext *aCx, JS::Handle<JSObject*> aGivenProto) override;
-
-  // The ShadowRoot that will be rendered in place of this shadow insertion point.
-  RefPtr<ShadowRoot> mProjectedShadow;
-
-  bool mIsInsertionPoint;
-};
-
-} // namespace dom
-} // namespace mozilla
-
-#endif // mozilla_dom_HTMLShadowElement_h__
-
diff --git a/dom/html/moz.build b/dom/html/moz.build
index c86c169b5c..1ccc630fdb 100644
--- a/dom/html/moz.build
+++ b/dom/html/moz.build
@@ -92,7 +92,6 @@ EXPORTS.mozilla.dom += [
     'HTMLProgressElement.h',
     'HTMLScriptElement.h',
     'HTMLSelectElement.h',
-    'HTMLShadowElement.h',
     'HTMLSharedElement.h',
     'HTMLSharedListElement.h',
     'HTMLSharedObjectElement.h',
@@ -172,7 +171,6 @@ UNIFIED_SOURCES += [
     'HTMLProgressElement.cpp',
     'HTMLScriptElement.cpp',
     'HTMLSelectElement.cpp',
-    'HTMLShadowElement.cpp',
     'HTMLSharedElement.cpp',
     'HTMLSharedListElement.cpp',
     'HTMLSharedObjectElement.cpp',
diff --git a/dom/html/nsGenericHTMLElement.h b/dom/html/nsGenericHTMLElement.h
index 16f438acc0..3b451b29cd 100644
--- a/dom/html/nsGenericHTMLElement.h
+++ b/dom/html/nsGenericHTMLElement.h
@@ -1700,7 +1700,6 @@ NS_DECLARE_NS_NEW_HTML_ELEMENT(Pre)
 NS_DECLARE_NS_NEW_HTML_ELEMENT(Progress)
 NS_DECLARE_NS_NEW_HTML_ELEMENT(Script)
 NS_DECLARE_NS_NEW_HTML_ELEMENT(Select)
-NS_DECLARE_NS_NEW_HTML_ELEMENT(Shadow)
 NS_DECLARE_NS_NEW_HTML_ELEMENT(Source)
 NS_DECLARE_NS_NEW_HTML_ELEMENT(Span)
 NS_DECLARE_NS_NEW_HTML_ELEMENT(Style)
diff --git a/dom/html/nsGenericHTMLFrameElement.cpp b/dom/html/nsGenericHTMLFrameElement.cpp
index 03e6ffb421..8170179e2c 100644
--- a/dom/html/nsGenericHTMLFrameElement.cpp
+++ b/dom/html/nsGenericHTMLFrameElement.cpp
@@ -27,6 +27,7 @@
 #include "nsServiceManagerUtils.h"
 #include "nsSubDocumentFrame.h"
 #include "nsXULElement.h"
+#include "nsAttrValueOrString.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
diff --git a/dom/html/nsHTMLContentSink.cpp b/dom/html/nsHTMLContentSink.cpp
index 1c390a53ec..eb22b772f9 100644
--- a/dom/html/nsHTMLContentSink.cpp
+++ b/dom/html/nsHTMLContentSink.cpp
@@ -31,6 +31,7 @@
 #include "mozilla/Logging.h"
 #include "nsNodeUtils.h"
 #include "nsIContent.h"
+#include "mozilla/dom/CustomElementRegistry.h"
 #include "mozilla/dom/Element.h"
 #include "mozilla/Preferences.h"
 
diff --git a/dom/tests/mochitest/webcomponents/mochitest.ini b/dom/tests/mochitest/webcomponents/mochitest.ini
index f5d0f84ea9..756e6f2ae1 100644
--- a/dom/tests/mochitest/webcomponents/mochitest.ini
+++ b/dom/tests/mochitest/webcomponents/mochitest.ini
@@ -25,7 +25,6 @@ skip-if = true || stylo # disabled - See bug 1390396 and 1293844
 skip-if = !debug # TestFunctions only applied in debug builds
 [test_nested_content_element.html]
 [test_dest_insertion_points.html]
-[test_dest_insertion_points_shadow.html]
 [test_fallback_dest_insertion_points.html]
 [test_detached_style.html]
 [test_dynamic_content_element_matching.html]
@@ -38,7 +37,6 @@ skip-if = true # disabled - See bug 1390396
 [test_document_register_stack.html]
 skip-if = true # disabled - See bug 1390396
 [test_document_shared_registry.html]
-[test_event_dispatch.html]
 [test_event_retarget.html]
 [test_event_stopping.html]
 [test_template.html]
@@ -46,11 +44,8 @@ skip-if = true # disabled - See bug 1390396
 [test_template_custom_elements.html]
 [test_shadowroot.html]
 [test_shadowroot_inert_element.html]
-[test_shadowroot_host.html]
 [test_shadowroot_style.html]
-[test_shadowroot_style_multiple_shadow.html]
 [test_shadowroot_style_order.html]
-[test_shadowroot_youngershadowroot.html]
 [test_style_fallback_content.html]
 [test_unresolved_pseudo_class.html]
 [test_link_prefetch.html]
diff --git a/dom/tests/mochitest/webcomponents/test_dest_insertion_points_shadow.html b/dom/tests/mochitest/webcomponents/test_dest_insertion_points_shadow.html
deleted file mode 100644
index 75286463e6..0000000000
--- a/dom/tests/mochitest/webcomponents/test_dest_insertion_points_shadow.html
+++ /dev/null
@@ -1,68 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<!--
-https://bugzilla.mozilla.org/show_bug.cgi?id=999999
--->
-<head>
-  <meta charset="utf-8">
-  <title>Test for Bug 999999</title>
-  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
-  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
-</head>
-<body>
-<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=999999">Mozilla Bug 999999</a>
-<p id="display"></p>
-<div id="content">
-<div id="shadowhost"></div>
-</div>
-<pre id="test">
-</pre>
-<script type="application/javascript">
-
-/** Test for Bug 999999 **/
-var host = document.getElementById("shadowhost");
-
-// Test destination insertion points of node distributed to shadow element.
-var olderShadowRoot = host.createShadowRoot();
-var youngerShadowRoot = host.createShadowRoot();
-
-var shadowElem = document.createElement("shadow");
-youngerShadowRoot.appendChild(shadowElem);
-
-var span = document.createElement("span");
-olderShadowRoot.appendChild(span);
-
-is(span.getDestinationInsertionPoints().length, 1, "Child of ShadowRoot should be distributed to shadow insertion point.");
-is(span.getDestinationInsertionPoints()[0], shadowElem, "Shadow element should be in destination insertion point list.");
-
-// Test destination insertion points of node removed from tree.
-olderShadowRoot.removeChild(span);
-is(span.getDestinationInsertionPoints().length, 0, "Node removed from tree should no longer be distributed.");
-
-// Test destination insertion points of fallback content being reprojected into a shadow element.
-var content = document.createElement("content");
-var fallback = document.createElement("span");
-
-content.appendChild(fallback);
-olderShadowRoot.appendChild(content);
-
-is(fallback.getDestinationInsertionPoints().length, 2, "The fallback content should have 2 destination insertion points, the parent content and the shadow element to which it is reprojected.");
-is(fallback.getDestinationInsertionPoints()[0], content, "First destination of the fallback content should be the parent content element.");
-is(fallback.getDestinationInsertionPoints()[1], shadowElem, "Second destination of the fallback content should be the shadow element to which the element is reprojected.");
-
-// Test destination insertion points of fallback content being removed from tree.
-content.removeChild(fallback);
-is(fallback.getDestinationInsertionPoints().length, 0, "The content should no longer be distributed to any nodes because it is no longer fallback content.");
-
-// Test destination insertion points of distributed content after removing shadow insertion point.
-var div = document.createElement("div");
-olderShadowRoot.appendChild(div);
-is(div.getDestinationInsertionPoints().length, 1, "Children in older shadow root should be distributed to shadow insertion point.");
-is(div.getDestinationInsertionPoints()[0], shadowElem, "Destination insertion point should include shadow element.");
-
-youngerShadowRoot.removeChild(shadowElem);
-is(div.getDestinationInsertionPoints().length, 0, "Destination insertion points should be empty after removing shadow element.");
-
-</script>
-</body>
-</html>
diff --git a/dom/tests/mochitest/webcomponents/test_event_dispatch.html b/dom/tests/mochitest/webcomponents/test_event_dispatch.html
deleted file mode 100644
index c73bfb214e..0000000000
--- a/dom/tests/mochitest/webcomponents/test_event_dispatch.html
+++ /dev/null
@@ -1,458 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<!--
-https://bugzilla.mozilla.org/show_bug.cgi?id=887541
--->
-<head>
-  <title>Test for event model in web components</title>
-  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
-  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
-</head>
-<body>
-<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=887541">Bug 887541</a>
-<script>
-
-var els = SpecialPowers.Cc["@mozilla.org/eventlistenerservice;1"]
-            .getService(SpecialPowers.Ci.nsIEventListenerService);
-
-function eventListener(e) {
-  eventChain.push(this);
-}
-
-function isEventChain(actual, expected, msg) {
-  is(actual.length, expected.length, msg);
-  for (var i = 0; i < expected.length; i++) {
-    is(actual[i], expected[i], msg + " at " + i);
-  }
-
-  // Check to make sure the event chain matches what we get back from nsIEventListenerService.getEventTargetChainFor
-  if (0 < actual.length) {
-    var chain = els.getEventTargetChainFor(actual[0], true); // Events should be dispatched on actual[0].
-    for (var i = 0; i < expected.length; i++) {
-      ok(SpecialPowers.compare(chain[i], expected[i]), msg + " at " + i + " for nsIEventListenerService");
-    }
-  }
-}
-
-/*
- * Test 1: Test of event dispatch through a basic ShadowRoot with content a insertion point.
- *
- * <div elemOne> ------ <shadow-root shadowOne>
- *        |                        |
- * <div elemTwo>            <span elemThree>
- *                                 |
- *                         <content elemFour>
- */
-
-var elemOne = document.createElement("div");
-elemOne.addEventListener("custom", eventListener);
-
-var elemTwo = document.createElement("div");
-elemTwo.addEventListener("custom", eventListener);
-
-var elemThree = document.createElement("span");
-elemThree.addEventListener("custom", eventListener);
-
-var elemFour = document.createElement("content");
-elemFour.addEventListener("custom", eventListener);
-
-var shadowOne = elemOne.createShadowRoot();
-shadowOne.addEventListener("custom", eventListener);
-
-elemThree.appendChild(elemFour);
-shadowOne.appendChild(elemThree);
-elemOne.appendChild(elemTwo);
-
-var eventChain = [];
-var customEvent = new CustomEvent("custom", { "bubbles" : true, "composed" : true });
-elemTwo.dispatchEvent(customEvent);
-isEventChain(eventChain, [elemTwo, elemFour, elemThree, shadowOne, elemOne], "Event path for test 1 for event dispatched on elemTwo.");
-
-/*
- * Test 2: Test of event dispatch through a nested ShadowRoots with content insertion points.
- *
- * <div elemFive> --- <shadow-root shadowTwo>
- *       |                       |
- * <div elemOne>          <div elemFour> ----- <shadow-root shadowOne>
- *                               |                        |
- *                       <content elemTwo>           <p elemSix>
- *                                                        |
- *                                               <content elemThree>
- */
-
-elemOne = document.createElement("div");
-elemOne.addEventListener("custom", eventListener);
-
-elemTwo = document.createElement("content");
-elemTwo.addEventListener("custom", eventListener);
-
-elemThree = document.createElement("content");
-elemThree.addEventListener("custom", eventListener);
-
-var elemFour = document.createElement("div");
-elemFour.addEventListener("custom", eventListener);
-
-var elemFive = document.createElement("div");
-elemFive.addEventListener("custom", eventListener);
-
-var elemSix = document.createElement("p");
-elemSix.addEventListener("custom", eventListener);
-
-var shadowOne = elemFour.createShadowRoot();
-shadowOne.addEventListener("custom", eventListener);
-
-var shadowTwo = elemFive.createShadowRoot();
-shadowTwo.addEventListener("custom", eventListener);
-
-elemFive.appendChild(elemOne);
-shadowTwo.appendChild(elemFour);
-elemFour.appendChild(elemTwo);
-shadowOne.appendChild(elemSix);
-elemSix.appendChild(elemThree);
-
-eventChain = [];
-customEvent = new CustomEvent("custom", { "bubbles" : true, "composed" : true });
-elemOne.dispatchEvent(customEvent);
-is(elemOne.getDestinationInsertionPoints().length, 2, "yes");
-isEventChain(eventChain, [elemOne, elemThree, elemSix, shadowOne, elemTwo, elemFour, shadowTwo, elemFive], "Event path for test 2 for event dispatched on elemOne.");
-
-/*
- * Test 3: Test of event dispatch through nested ShadowRoot with content insertion points.
- *
- * <div elemOne> ------- <shadow-root shadowOne>
- *        |                        |
- * <span elemTwo>          <span elemThree> ------------ <shadow-root shadowTwo>
- *                                 |                               |
- *                          <span elemFour>                <content elemSix>
- *                                 |
- *                         <content elemFive>
- */
-
-elemOne = document.createElement("div");
-elemOne.addEventListener("custom", eventListener);
-
-elemTwo = document.createElement("span");
-elemTwo.addEventListener("custom", eventListener);
-
-elemThree = document.createElement("span");
-elemThree.addEventListener("custom", eventListener);
-
-elemFour = document.createElement("span");
-elemFour.addEventListener("custom", eventListener);
-
-elemFive = document.createElement("content");
-elemFive.addEventListener("custom", eventListener);
-
-elemSix = document.createElement("content");
-elemSix.addEventListener("custom", eventListener);
-
-shadowOne = elemOne.createShadowRoot();
-shadowOne.addEventListener("custom", eventListener);
-
-shadowTwo = elemThree.createShadowRoot();
-shadowTwo.addEventListener("custom", eventListener);
-
-elemOne.appendChild(elemTwo);
-shadowOne.appendChild(elemThree);
-elemThree.appendChild(elemFour);
-elemFour.appendChild(elemFive);
-shadowTwo.appendChild(elemSix);
-
-eventChain = [];
-customEvent = new CustomEvent("custom", { "bubbles" : true, "composed" : true });
-elemTwo.dispatchEvent(customEvent);
-isEventChain(eventChain, [elemTwo, elemFive, elemFour, elemSix, shadowTwo, elemThree, shadowOne, elemOne], "Event path for test 3 for event dispatched on elemTwo.");
-
-/*
- * Test 4: Test of event dispatch through host with multiple ShadowRoots with shadow insertion point.
- *
- * <div elemSeven> --- <shadow-root shadowTwo> (younger ShadowRoot)
- *       |         |             |
- * <div elemOne>   |      <div elemSix> -------- <shadow-root shadowOne>
- *                 |             |                         |
- *                 |     <shadow elemFour>         <content elemFive>
- *                 |             |
- *                 |     <content elemTwo>
- *                 |
- *                 --- <shadow-root shadowThree> (older ShadowRoot)
- *                         |                |
- *                         |       <content elemThree>
- *                         |
- *                  <div elemEight>
- */
-
-elemOne = document.createElement("div");
-elemOne.addEventListener("custom", eventListener);
-
-elemTwo = document.createElement("content");
-elemTwo.addEventListener("custom", eventListener);
-
-elemThree = document.createElement("content");
-elemThree.addEventListener("custom", eventListener);
-
-elemFour = document.createElement("shadow");
-elemFour.addEventListener("custom", eventListener);
-
-elemFive = document.createElement("content");
-elemFive.addEventListener("custom", eventListener);
-
-elemSix = document.createElement("div");
-elemSix.addEventListener("custom", eventListener);
-
-var elemSeven = document.createElement("div");
-elemSeven.addEventListener("custom", eventListener);
-
-var elemEight = document.createElement("div");
-elemEight.addEventListener("custom", eventListener);
-
-var shadowThree = elemSeven.createShadowRoot();
-shadowThree.addEventListener("custom", eventListener);
-
-shadowTwo = elemSeven.createShadowRoot();
-shadowTwo.addEventListener("custom", eventListener);
-
-shadowOne = elemSix.createShadowRoot();
-shadowOne.addEventListener("custom", eventListener);
-
-elemSeven.appendChild(elemOne);
-shadowTwo.appendChild(elemSix);
-elemSix.appendChild(elemFour);
-elemFour.appendChild(elemTwo);
-shadowThree.appendChild(elemEight);
-shadowThree.appendChild(elemThree);
-shadowOne.appendChild(elemFive);
-
-eventChain = [];
-customEvent = new CustomEvent("custom", { "bubbles" : true, "composed" : true });
-elemOne.dispatchEvent(customEvent);
-isEventChain(eventChain, [elemOne, elemFive, shadowOne, elemThree, shadowThree, elemTwo, elemFour, elemSix, shadowTwo, elemSeven], "Event path for test 4 for event dispatched on elemOne.");
-
-eventChain = [];
-customEvent = new CustomEvent("custom", { "bubbles" : true, "composed" : true });
-elemEight.dispatchEvent(customEvent);
-isEventChain(eventChain, [elemEight, elemFive, shadowOne, elemSix, shadowTwo, elemSeven], "Event path for test 4 for event dispatched on elemEight.");
-
-/*
- * Test 5: Test of event dispatch through nested shadowroot with insertion points that match specific tags.
- *
- * <div elemOne> --------- <shadow-root shadowOne>
- *    |      |                        |
- *    |  <p elemThree>        <span elemFour> ------------------------ <shadow-root shadowTwo>
- *    |                          |       |                                        |
- * <span elemTwo>                |   <content select="p" elemFive>       <content elemSeven>
- *                               |
- *                       <content select="span" elemSix>
- */
-
-elemOne = document.createElement("div");
-elemOne.addEventListener("custom", eventListener);
-
-elemTwo = document.createElement("span");
-elemTwo.addEventListener("custom", eventListener);
-
-elemThree = document.createElement("p");
-elemThree.addEventListener("custom", eventListener);
-
-elemFour = document.createElement("span");
-elemFour.addEventListener("custom", eventListener);
-
-elemFive = document.createElement("content");
-elemFive.select = "p";
-elemFive.addEventListener("custom", eventListener);
-
-elemSix = document.createElement("content");
-elemSix.select = "span";
-elemSix.addEventListener("custom", eventListener);
-
-elemSeven = document.createElement("content");
-elemSeven.addEventListener("custom", eventListener);
-
-shadowTwo = elemFour.createShadowRoot();
-shadowTwo.addEventListener("custom", eventListener);
-
-shadowOne = elemOne.createShadowRoot();
-shadowOne.addEventListener("custom", eventListener);
-
-elemOne.appendChild(elemTwo);
-elemOne.appendChild(elemThree);
-shadowOne.appendChild(elemFour);
-elemFour.appendChild(elemSix);
-elemFour.appendChild(elemFive);
-shadowTwo.appendChild(elemSeven);
-
-eventChain = [];
-customEvent = new CustomEvent("custom", { "bubbles" : true, "composed" : true });
-elemTwo.dispatchEvent(customEvent);
-isEventChain(eventChain, [elemTwo, elemSeven, shadowTwo, elemSix, elemFour, shadowOne, elemOne], "Event path for test 5 for event dispatched on elemTwo.");
-
-eventChain = [];
-customEvent = new CustomEvent("custom", { "bubbles" : true, "composed" : true });
-elemThree.dispatchEvent(customEvent);
-isEventChain(eventChain, [elemThree, elemSeven, shadowTwo, elemFive, elemFour, shadowOne, elemOne], "Event path for test 5 for event dispatched on elemThree.");
-
-/*
- * Test 6: Test of event dispatch through nested shadowroot with insertion points that match specific tags.
- *
- * <div elemOne> --------- <shadow-root shadowOne>;
- *    |      |                        |
- *    |  <p elemThree>         <span elemFour> ------ <shadow-root shadowTwo>
- *    |                               |                   |            |
- * <span elemTwo>            <content elemFive>           |  <content select="p" elemSeven>
- *                                                        |
- *                                                 <content select="span" elemSix>
- */
-
-elemOne = document.createElement("div");
-elemOne.addEventListener("custom", eventListener);
-
-elemTwo = document.createElement("span");
-elemTwo.addEventListener("custom", eventListener);
-
-elemThree = document.createElement("p");
-elemThree.addEventListener("custom", eventListener);
-
-elemFour = document.createElement("span");
-elemFour.addEventListener("custom", eventListener);
-
-elemFive = document.createElement("content");
-elemFive.addEventListener("custom", eventListener);
-
-elemSix = document.createElement("content");
-elemSix.select = "span";
-elemSix.addEventListener("custom", eventListener);
-
-elemSeven = document.createElement("content");
-elemSeven.select = "p";
-elemSeven.addEventListener("custom", eventListener);
-
-shadowTwo = elemFour.createShadowRoot();
-shadowTwo.addEventListener("custom", eventListener);
-
-shadowOne = elemOne.createShadowRoot();
-shadowOne.addEventListener("custom", eventListener);
-
-elemOne.appendChild(elemTwo);
-elemOne.appendChild(elemThree);
-shadowOne.appendChild(elemFour);
-elemFour.appendChild(elemFive);
-shadowTwo.appendChild(elemSix);
-shadowTwo.appendChild(elemSeven);
-
-eventChain = [];
-customEvent = new CustomEvent("custom", { "bubbles" : true, "composed" : true });
-elemTwo.dispatchEvent(customEvent);
-isEventChain(eventChain, [elemTwo, elemSix, shadowTwo, elemFive, elemFour, shadowOne, elemOne], "Event path for test 6 for event dispatched on elemTwo.");
-
-eventChain = [];
-customEvent = new CustomEvent("custom", { "bubbles" : true, "composed" : true });
-elemThree.dispatchEvent(customEvent);
-isEventChain(eventChain, [elemThree, elemSeven, shadowTwo, elemFive, elemFour, shadowOne, elemOne], "Event path for test 6 for event dispatched on elemThree.");
-
-/*
- * Test 7: Test of event dispatch through nested shadowroot with insertion points that match specific tags.
- *
- * <div elemOne> --------- <shadow-root shadowOne>
- *    |      |                        |
- *    |  <p elemThree>         <span elemFour> ------ <shadow-root shadowTwo>
- *    |                               |                         |
- * <span elemTwo>            <content elemFive>           <span elemEight>
- *                                                           |        |
- *                                                           |   <content select="p" elemSeven>
- *                                                           |
- *                                              <content select="span" elemSix>
- */
-
-elemOne = document.createElement("div");
-elemOne.addEventListener("custom", eventListener);
-
-elemTwo = document.createElement("span");
-elemTwo.addEventListener("custom", eventListener);
-
-elemThree = document.createElement("p");
-elemThree.addEventListener("custom", eventListener);
-
-elemFour = document.createElement("span");
-elemFour.addEventListener("custom", eventListener);
-
-elemFive = document.createElement("content");
-elemFive.addEventListener("custom", eventListener);
-
-elemSix = document.createElement("content");
-elemSix.select = "span";
-elemSix.addEventListener("custom", eventListener);
-
-elemSeven = document.createElement("content");
-elemSeven.select = "p";
-elemSeven.addEventListener("custom", eventListener);
-
-elemEight = document.createElement("span");
-elemEight.addEventListener("custom", eventListener);
-
-shadowTwo = elemFour.createShadowRoot();
-shadowTwo.addEventListener("custom", eventListener);
-
-shadowOne = elemOne.createShadowRoot();
-shadowOne.addEventListener("custom", eventListener);
-
-elemOne.appendChild(elemTwo);
-elemOne.appendChild(elemThree);
-shadowOne.appendChild(elemFour);
-elemFour.appendChild(elemFive);
-shadowTwo.appendChild(elemEight);
-elemEight.appendChild(elemSix);
-elemEight.appendChild(elemSeven);
-
-eventChain = [];
-customEvent = new CustomEvent("custom", { "bubbles" : true, "composed" : true });
-elemTwo.dispatchEvent(customEvent);
-isEventChain(eventChain, [elemTwo, elemSix, elemEight, shadowTwo, elemFive, elemFour, shadowOne, elemOne], "Event path for test 7 for event dispatched on elemTwo.");
-
-eventChain = [];
-customEvent = new CustomEvent("custom", { "bubbles" : true, "composed" : true });
-elemThree.dispatchEvent(customEvent);
-isEventChain(eventChain, [elemThree, elemSeven, elemEight, shadowTwo, elemFive, elemFour, shadowOne, elemOne], "Event path for test 7 for event dispatched on elemThree.");
-
-/*
- * Test 8: Test of event dispatch through host with multiple ShadowRoots with shadow insertion point.
- *
- * <div elemOne> --- <shadow-root shadowOne> (younger ShadowRoot)
- *               |             |
- *               |      <div elemFour>
- *               |             |
- *               |     <shadow elemTwo>
- *               |
- *               --- <shadow-root shadowTwo> (older ShadowRoot)
- *                             |
- *                      <div elemThree>
- */
-
-elemOne = document.createElement("div");
-elemOne.addEventListener("custom", eventListener);
-
-elemTwo = document.createElement("shadow");
-elemTwo.addEventListener("custom", eventListener);
-
-elemThree = document.createElement("div");
-elemThree.addEventListener("custom", eventListener);
-
-elemFour = document.createElement("div");
-elemFour.addEventListener("custom", eventListener);
-
-shadowTwo = elemOne.createShadowRoot();
-shadowTwo.addEventListener("custom", eventListener);
-
-shadowOne = elemOne.createShadowRoot();
-shadowOne.addEventListener("custom", eventListener);
-
-shadowOne.appendChild(elemFour);
-elemFour.appendChild(elemTwo);
-shadowTwo.appendChild(elemThree);
-
-eventChain = [];
-customEvent = new CustomEvent("custom", { "bubbles" : true, "composed" : true });
-elemThree.dispatchEvent(customEvent);
-isEventChain(eventChain, [elemThree, shadowTwo, elemTwo, elemFour, shadowOne, elemOne], "Event path for test 8 for event dispatched on elemThree.");
-
-</script>
-</body>
-</html>
diff --git a/dom/tests/mochitest/webcomponents/test_shadowroot_host.html b/dom/tests/mochitest/webcomponents/test_shadowroot_host.html
deleted file mode 100644
index f48d63e87d..0000000000
--- a/dom/tests/mochitest/webcomponents/test_shadowroot_host.html
+++ /dev/null
@@ -1,41 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<!--
-https://bugzilla.mozilla.org/show_bug.cgi?id=1083587
--->
-<head>
-  <meta charset="utf-8">
-  <title>Test for Bug 1083587</title>
-  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
-  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
-</head>
-<body>
-<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1083587">Mozilla Bug 1083587</a>
-<p id="display"></p>
-<div id="content" style="display: none">
-<div id="host"></div>
-</div>
-<pre id="test">
-</pre>
-<script type="application/javascript">
-
-/** Test for Bug 1083587 **/
-var hostInDoc = document.getElementById("host");
-var shadowOne = hostInDoc.createShadowRoot();
-is(shadowOne.host, hostInDoc);
-
-var shadowTwo = hostInDoc.createShadowRoot();
-is(shadowOne.host, hostInDoc);
-is(shadowTwo.host, hostInDoc);
-
-var hostNotInDoc = document.createElement("div");
-var shadowThree = hostNotInDoc.createShadowRoot();
-is(shadowThree.host, hostNotInDoc);
-
-var shadowFour = hostNotInDoc.createShadowRoot();
-is(shadowThree.host, hostNotInDoc);
-is(shadowFour.host, hostNotInDoc);
-
-</script>
-</body>
-</html>
diff --git a/dom/tests/mochitest/webcomponents/test_shadowroot_style_multiple_shadow.html b/dom/tests/mochitest/webcomponents/test_shadowroot_style_multiple_shadow.html
deleted file mode 100644
index 7a606bcd7f..0000000000
--- a/dom/tests/mochitest/webcomponents/test_shadowroot_style_multiple_shadow.html
+++ /dev/null
@@ -1,57 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<!--
-https://bugzilla.mozilla.org/show_bug.cgi?id=806506
--->
-<head>
-  <title>Test for ShadowRoot styles with multiple ShadowRoot on host.</title>
-  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
-  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
-</head>
-<body>
-<div class="tall" id="bodydiv"></div>
-<div id="container"></div>
-<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=806506">Bug 806506</a>
-<script>
-// Create ShadowRoot.
-var container = document.getElementById("container");
-var elem = document.createElement("div");
-container.appendChild(elem); // Put ShadowRoot host in document.
-var firstRoot = elem.createShadowRoot();
-var secondRoot = elem.createShadowRoot();
-var thirdRoot = elem.createShadowRoot();
-
-// A style element that will be appended into the ShadowRoot.
-var firstStyle = document.createElement("style");
-firstRoot.appendChild(firstStyle);
-is(firstRoot.styleSheets.length, 1, "firstStyle should be the only style in firstRoot.");
-is(firstRoot.styleSheets[0].ownerNode, firstStyle, "firstStyle should in the ShadowRoot styleSheets.");
-
-var secondStyle = document.createElement("style");
-secondRoot.appendChild(secondStyle);
-is(secondRoot.styleSheets.length, 1, "secondStyle should be the only style in secondRoot.");
-is(secondRoot.styleSheets[0].ownerNode, secondStyle, "secondStyle should in the ShadowRoot styleSheets.");
-
-var thirdStyle = document.createElement("style");
-thirdRoot.appendChild(thirdStyle);
-is(thirdRoot.styleSheets.length, 1, "thirdStyle should be the only style in thirdRoot.");
-is(thirdRoot.styleSheets[0].ownerNode, thirdStyle, "thirdStyle should in the ShadowRoot styleSheets.");
-
-// Check the stylesheet counts again to make sure that none of the style sheets leaked into the older ShadowRoots.
-is(firstRoot.styleSheets.length, 1, "Adding a stylesheet to a younger ShadowRoot should not affect stylesheets in the older ShadowRoot.");
-is(secondRoot.styleSheets.length, 1, "Adding a stylesheet to a younger ShadowRoot should not affect stylesheets in the older ShadowRoot.");
-
-// Remove styles and make sure they are removed from the correct ShadowRoot.
-firstRoot.removeChild(firstStyle);
-is(firstRoot.styleSheets.length, 0, "firstRoot should no longer have any styles.");
-
-thirdRoot.removeChild(thirdStyle);
-is(thirdRoot.styleSheets.length, 0, "thirdRoot should no longer have any styles.");
-
-secondRoot.removeChild(secondStyle);
-is(secondRoot.styleSheets.length, 0, "secondRoot should no longer have any styles.");
-
-</script>
-</body>
-</html>
-
diff --git a/dom/tests/mochitest/webcomponents/test_shadowroot_youngershadowroot.html b/dom/tests/mochitest/webcomponents/test_shadowroot_youngershadowroot.html
deleted file mode 100644
index 17743321bc..0000000000
--- a/dom/tests/mochitest/webcomponents/test_shadowroot_youngershadowroot.html
+++ /dev/null
@@ -1,41 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<!--
-https://bugzilla.mozilla.org/show_bug.cgi?id=1083587
--->
-<head>
-  <meta charset="utf-8">
-  <title>Test for Bug 1083587</title>
-  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
-  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
-</head>
-<body>
-<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1083587">Mozilla Bug 1083587</a>
-<p id="display"></p>
-<div id="content" style="display: none">
-<div id="host"></div>
-</div>
-<pre id="test">
-</pre>
-<script type="application/javascript">
-
-/** Test for Bug 1083587 **/
-var hostInDoc = document.getElementById("host");
-var shadowOne = hostInDoc.createShadowRoot();
-is(shadowOne.olderShadowRoot, null);
-
-var shadowTwo = hostInDoc.createShadowRoot();
-is(shadowOne.olderShadowRoot, null);
-is(shadowTwo.olderShadowRoot, shadowOne);
-
-var hostNotInDoc = document.createElement("div");
-var shadowThree = hostNotInDoc.createShadowRoot();
-is(shadowThree.olderShadowRoot, null);
-
-var shadowFour = hostNotInDoc.createShadowRoot();
-is(shadowThree.olderShadowRoot, null);
-is(shadowFour.olderShadowRoot, shadowThree);
-
-</script>
-</body>
-</html>
diff --git a/dom/webidl/HTMLShadowElement.webidl b/dom/webidl/HTMLShadowElement.webidl
deleted file mode 100644
index f72cd06e18..0000000000
--- a/dom/webidl/HTMLShadowElement.webidl
+++ /dev/null
@@ -1,19 +0,0 @@
-/* -*- 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://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html
- *
- * © Copyright 2004-2011 Apple Computer, Inc., Mozilla Foundation, and
- * Opera Software ASA. You are granted a license to use, reproduce
- * and create derivative works of this document.
- */
-
-[Func="nsDocument::IsWebComponentsEnabled"]
-interface HTMLShadowElement : HTMLElement
-{
-  readonly attribute ShadowRoot? olderShadowRoot;
-};
-
diff --git a/dom/webidl/ShadowRoot.webidl b/dom/webidl/ShadowRoot.webidl
index 8dd069244c..90e6b64676 100644
--- a/dom/webidl/ShadowRoot.webidl
+++ b/dom/webidl/ShadowRoot.webidl
@@ -20,7 +20,6 @@ interface ShadowRoot : DocumentFragment
   [CEReactions, SetterThrows, TreatNullAs=EmptyString]
   attribute DOMString innerHTML;
   readonly attribute Element host;
-  readonly attribute ShadowRoot? olderShadowRoot;
   attribute boolean applyAuthorStyles;
   readonly attribute StyleSheetList styleSheets;
 };
diff --git a/dom/webidl/moz.build b/dom/webidl/moz.build
index 156da302dd..dba8e2818e 100644
--- a/dom/webidl/moz.build
+++ b/dom/webidl/moz.build
@@ -226,7 +226,6 @@ WEBIDL_FILES = [
     'HTMLQuoteElement.webidl',
     'HTMLScriptElement.webidl',
     'HTMLSelectElement.webidl',
-    'HTMLShadowElement.webidl',
     'HTMLSourceElement.webidl',
     'HTMLSpanElement.webidl',
     'HTMLStyleElement.webidl',