Bug 1409975 - Implement node distribution for shadow tree slots

* Implementation for assignedNodes
* Include slots in the flat tree
* Fix event get-the-parent algorithm for a node
* Update and add reftests for Shadow DOM v1
* Update web platform tests expectations

Tag #1375

49 files changed, 775 insertions, 429 deletions

diff --git a/dom/base/ChildIterator.cpp b/dom/base/ChildIterator.cpp
index dc356fc01..ae5f4665d 100644
--- a/dom/base/ChildIterator.cpp
+++ b/dom/base/ChildIterator.cpp
@@ -6,6 +6,7 @@
 
 #include "ChildIterator.h"
 #include "nsContentUtils.h"
+#include "mozilla/dom/HTMLSlotElement.h"
 #include "mozilla/dom/XBLChildrenElement.h"
 #include "mozilla/dom/ShadowRoot.h"
 #include "nsIAnonymousContentCreator.h"
@@ -57,15 +58,34 @@ GetMatchedNodesForPoint(nsIContent* aContent)
   // XXX handle <slot> element?
 }
 
+ExplicitChildIterator::ExplicitChildIterator(const nsIContent* aParent,
+                                             bool aStartAtBeginning)
+  : mParent(aParent),
+    mChild(nullptr),
+    mDefaultChild(nullptr),
+    mIsFirst(aStartAtBeginning),
+    mIndexInInserted(0)
+{
+  mParentAsSlot = HTMLSlotElement::FromContent(mParent);
+}
+
 nsIContent*
 ExplicitChildIterator::GetNextChild()
 {
   // If we're already in the inserted-children array, look there first
   if (mIndexInInserted) {
     MOZ_ASSERT(mChild);
-    MOZ_ASSERT(nsContentUtils::IsContentInsertionPoint(mChild));
     MOZ_ASSERT(!mDefaultChild);
 
+    if (mParentAsSlot) {
+      const nsTArray<RefPtr<nsINode>>& assignedNodes =
+        mParentAsSlot->AssignedNodes();
+
+      mChild = (mIndexInInserted < assignedNodes.Length()) ?
+        assignedNodes[mIndexInInserted++]->AsContent() : nullptr;
+      return mChild;
+    }
+
     MatchedNodes assignedChildren = GetMatchedNodesForPoint(mChild);
     if (mIndexInInserted < assignedChildren.Length()) {
       return assignedChildren[mIndexInInserted++];
@@ -84,6 +104,19 @@ ExplicitChildIterator::GetNextChild()
 
     mChild = mChild->GetNextSibling();
   } else if (mIsFirst) {  // at the beginning of the child list
+    // For slot parent, iterate over assigned nodes if not empty, otherwise
+    // fall through and iterate over direct children (fallback content).
+    if (mParentAsSlot) {
+      const nsTArray<RefPtr<nsINode>>& assignedNodes =
+        mParentAsSlot->AssignedNodes();
+      if (!assignedNodes.IsEmpty()) {
+        mIndexInInserted = 1;
+        mChild = assignedNodes[0]->AsContent();
+        mIsFirst = false;
+        return mChild;
+      }
+    }
+
     mChild = mParent->GetFirstChild();
     mIsFirst = false;
   } else if (mChild) { // in the middle of the child list
@@ -185,6 +218,12 @@ ExplicitChildIterator::Get() const
 {
   MOZ_ASSERT(!mIsFirst);
 
+  // When mParentAsSlot is set, mChild is always set to the current child. It
+  // does not matter whether mChild is an assigned node or a fallback content.
+  if (mParentAsSlot) {
+    return mChild;
+  }
+
   if (mIndexInInserted) {
     MatchedNodes assignedChildren = GetMatchedNodesForPoint(mChild);
     return assignedChildren[mIndexInInserted - 1];
@@ -198,6 +237,20 @@ ExplicitChildIterator::GetPreviousChild()
 {
   // If we're already in the inserted-children array, look there first
   if (mIndexInInserted) {
+
+    if (mParentAsSlot) {
+      const nsTArray<RefPtr<nsINode>>& assignedNodes =
+        mParentAsSlot->AssignedNodes();
+
+      mChild = (--mIndexInInserted) ?
+        assignedNodes[mIndexInInserted - 1]->AsContent() : nullptr;
+
+      if (!mChild) {
+        mIsFirst = true;
+      }
+      return mChild;
+    }
+
     // NB: mIndexInInserted points one past the last returned child so we need
     // to look *two* indices back in order to return the previous child.
     MatchedNodes assignedChildren = GetMatchedNodesForPoint(mChild);
@@ -218,6 +271,18 @@ ExplicitChildIterator::GetPreviousChild()
   } else if (mChild) { // in the middle of the child list
     mChild = mChild->GetPreviousSibling();
   } else { // at the end of the child list
+    // For slot parent, iterate over assigned nodes if not empty, otherwise
+    // fall through and iterate over direct children (fallback content).
+    if (mParentAsSlot) {
+      const nsTArray<RefPtr<nsINode>>& assignedNodes =
+        mParentAsSlot->AssignedNodes();
+      if (!assignedNodes.IsEmpty()) {
+        mIndexInInserted = assignedNodes.Length();
+        mChild = assignedNodes[mIndexInInserted - 1]->AsContent();
+        return mChild;
+      }
+    }
+
     mChild = mParent->GetLastChild();
   }
 
diff --git a/dom/base/ChildIterator.h b/dom/base/ChildIterator.h
index f78ba7ca3..ec6dfa61d 100644
--- a/dom/base/ChildIterator.h
+++ b/dom/base/ChildIterator.h
@@ -36,22 +36,19 @@ class ExplicitChildIterator
 {
 public:
   explicit ExplicitChildIterator(const nsIContent* aParent,
-                                 bool aStartAtBeginning = true)
-    : mParent(aParent),
-      mChild(nullptr),
-      mDefaultChild(nullptr),
-      mIndexInInserted(0),
-      mIsFirst(aStartAtBeginning)
-  {
-  }
+                                 bool aStartAtBeginning = true);
 
   ExplicitChildIterator(const ExplicitChildIterator& aOther)
-    : mParent(aOther.mParent), mChild(aOther.mChild),
+    : mParent(aOther.mParent),
+      mParentAsSlot(aOther.mParentAsSlot),
+      mChild(aOther.mChild),
       mDefaultChild(aOther.mDefaultChild),
       mIndexInInserted(aOther.mIndexInInserted), mIsFirst(aOther.mIsFirst) {}
 
   ExplicitChildIterator(ExplicitChildIterator&& aOther)
-    : mParent(aOther.mParent), mChild(aOther.mChild),
+    : mParent(aOther.mParent),
+      mParentAsSlot(aOther.mParentAsSlot),
+      mChild(aOther.mChild),
       mDefaultChild(aOther.mDefaultChild),
       mIndexInInserted(aOther.mIndexInInserted), mIsFirst(aOther.mIsFirst) {}
 
@@ -98,6 +95,10 @@ protected:
   // the <xbl:content> element for the binding.
   const nsIContent* mParent;
 
+  // If parent is a slot element, this points to the parent as HTMLSlotElement,
+  // otherwise, it's null.
+  const HTMLSlotElement* mParentAsSlot;
+
   // The current child. When we encounter an insertion point,
   // mChild remains as the insertion point whose content we're iterating (and
   // our state is controled by mDefaultChild or mIndexInInserted depending on
diff --git a/dom/base/FragmentOrElement.cpp b/dom/base/FragmentOrElement.cpp
index 766e2b115..ecb18798f 100644
--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -183,6 +183,24 @@ nsIContent::GetAssignedSlotByMode() const
 }
 
 nsINode*
+nsIContent::GetFlattenedTreeParentForMaybeAssignedNode() const
+{
+  if (HTMLSlotElement* assignedSlot = GetAssignedSlot()) {
+    return assignedSlot;
+  }
+
+  HTMLSlotElement* parentSlot = HTMLSlotElement::FromContent(GetParent());
+  if (!parentSlot) {
+    return nullptr;
+  }
+
+  // If this is not an unassigned node, then it must be a fallback content.
+  MOZ_ASSERT(parentSlot->AssignedNodes().IsEmpty());
+
+  return parentSlot;
+}
+
+nsINode*
 nsIContent::GetFlattenedTreeParentNodeInternal(FlattenedParentType aType) const
 {
   nsINode* parentNode = GetParentNode();
@@ -233,16 +251,10 @@ nsIContent::GetFlattenedTreeParentNodeInternal(FlattenedParentType aType) const
 
   if (parent && nsContentUtils::HasDistributedChildren(parent) &&
       nsContentUtils::IsInSameAnonymousTree(parent, this)) {
-    // This node is distributed to insertion points, thus we
-    // need to consult the destination insertion points list to
-    // figure out where this node was inserted in the flattened tree.
-    // It may be the case that |parent| distributes its children
-    // but the child does not match any insertion points, thus
-    // the flattened tree parent is nullptr.
-    nsTArray<nsIContent*>* destInsertionPoints = GetExistingDestInsertionPoints();
-    parent = destInsertionPoints && !destInsertionPoints->IsEmpty() ?
-      destInsertionPoints->LastElement()->GetParent() : nullptr;
-  } else if (HasFlag(NODE_MAY_BE_IN_BINDING_MNGR)) {
+    return GetFlattenedTreeParentForMaybeAssignedNode();
+  }
+
+  if (HasFlag(NODE_MAY_BE_IN_BINDING_MNGR)) {
     nsIContent* insertionParent = GetXBLInsertionParent();
     if (insertionParent) {
       parent = insertionParent;
@@ -890,42 +902,10 @@ nsIContent::GetEventTargetParent(EventChainPreVisitor& aVisitor)
     }
   }
 
-  nsIContent* parent = GetParent();
-
-  // Web components have a special event chain that need to account
-  // for destination insertion points where nodes have been distributed.
-  nsTArray<nsIContent*>* destPoints = GetExistingDestInsertionPoints();
-  if (destPoints && !destPoints->IsEmpty()) {
-    // Push destination insertion points to aVisitor.mDestInsertionPoints.
-    for (uint32_t i = 0; i < destPoints->Length(); i++) {
-      nsIContent* point = destPoints->ElementAt(i);
-      aVisitor.mDestInsertionPoints.AppendElement(point);
-    }
-  }
-
-  ShadowRoot* thisShadowRoot = ShadowRoot::FromNode(this);
-  if (thisShadowRoot) {
-    if (!aVisitor.mEvent->mFlags.mComposed) {
-      // If we do stop propagation, we still want to propagate
-      // the event to chrome (nsPIDOMWindow::GetParentTarget()).
-      // The load event is special in that we don't ever propagate it
-      // to chrome.
-      nsCOMPtr<nsPIDOMWindowOuter> win = OwnerDoc()->GetWindow();
-      EventTarget* parentTarget = win && aVisitor.mEvent->mMessage != eLoad
-        ? win->GetParentTarget() : nullptr;
-
-      aVisitor.mParentTarget = parentTarget;
-      return NS_OK;
-    }
-
-    if (!aVisitor.mDestInsertionPoints.IsEmpty()) {
-      parent = aVisitor.mDestInsertionPoints.LastElement();
-      aVisitor.mDestInsertionPoints.SetLength(
-        aVisitor.mDestInsertionPoints.Length() - 1);
-    } else {
-      parent = thisShadowRoot->GetHost();
-    }
-  }
+  // Event parent is the assigned slot, if node is assigned, or node's parent
+  // otherwise.
+  HTMLSlotElement* slot = GetAssignedSlot();
+  nsIContent* parent = slot ? slot : GetParent();
 
   // Event may need to be retargeted if this is the root of a native
   // anonymous content subtree or event is dispatched somewhere inside XBL.
diff --git a/dom/base/ShadowRoot.cpp b/dom/base/ShadowRoot.cpp
index 97214e050..308f57cf7 100644
--- a/dom/base/ShadowRoot.cpp
+++ b/dom/base/ShadowRoot.cpp
@@ -14,7 +14,9 @@
 #include "nsIDOMHTMLElement.h"
 #include "nsIStyleSheetLinkingElement.h"
 #include "mozilla/dom/Element.h"
+#include "mozilla/dom/HTMLSlotElement.h"
 #include "nsXBLPrototypeBinding.h"
+#include "mozilla/EventDispatcher.h"
 #include "mozilla/StyleSheet.h"
 #include "mozilla/StyleSheetInlines.h"
 
@@ -110,6 +112,87 @@ ShadowRoot::FromNode(nsINode* aNode)
 }
 
 void
+ShadowRoot::AddSlot(HTMLSlotElement* aSlot)
+{
+  MOZ_ASSERT(aSlot);
+
+  // Note that if name attribute missing, the slot is a default slot.
+ nsAutoString name;
+  aSlot->GetName(name);
+
+  nsTArray<HTMLSlotElement*>* currentSlots = mSlotMap.LookupOrAdd(name);
+  MOZ_ASSERT(currentSlots);
+
+  HTMLSlotElement* oldSlot = currentSlots->IsEmpty() ?
+    nullptr : currentSlots->ElementAt(0);
+
+  TreeOrderComparator comparator;
+  currentSlots->InsertElementSorted(aSlot, comparator);
+
+  HTMLSlotElement* currentSlot = currentSlots->ElementAt(0);
+  if (currentSlot != aSlot) {
+    return;
+  }
+
+  if (oldSlot && oldSlot != currentSlot) {
+    // Move assigned nodes from old slot to new slot.
+    const nsTArray<RefPtr<nsINode>>& assignedNodes = oldSlot->AssignedNodes();
+    while (assignedNodes.Length() > 0) {
+      nsINode* assignedNode = assignedNodes[0];
+
+      oldSlot->RemoveAssignedNode(assignedNode);
+      currentSlot->AppendAssignedNode(assignedNode);
+    }
+  } else {
+    // Otherwise add appropriate nodes to this slot from the host.
+    for (nsIContent* child = GetHost()->GetFirstChild();
+         child;
+         child = child->GetNextSibling()) {
+      nsAutoString slotName;
+      child->GetAttr(kNameSpaceID_None, nsGkAtoms::slot, slotName);
+      if (child->IsSlotable() && slotName.Equals(name)) {
+        currentSlot->AppendAssignedNode(child);
+      }
+    }
+  }
+}
+
+void
+ShadowRoot::RemoveSlot(HTMLSlotElement* aSlot)
+{
+  MOZ_ASSERT(aSlot);
+
+  nsAutoString name;
+  aSlot->GetName(name);
+
+  nsTArray<HTMLSlotElement*>* currentSlots = mSlotMap.Get(name);
+
+  if (currentSlots) {
+    if (currentSlots->Length() == 1) {
+      MOZ_ASSERT(currentSlots->ElementAt(0) == aSlot);
+      mSlotMap.Remove(name);
+      aSlot->ClearAssignedNodes();
+    } else {
+      bool doReplaceSlot = currentSlots->ElementAt(0) == aSlot;
+      currentSlots->RemoveElement(aSlot);
+      HTMLSlotElement* replacementSlot = currentSlots->ElementAt(0);
+
+      // Move assigned nodes from removed slot to the next slot in
+      // tree order with the same name.
+      if (doReplaceSlot) {
+        const nsTArray<RefPtr<nsINode>>& assignedNodes = aSlot->AssignedNodes();
+        while (assignedNodes.Length() > 0) {
+          nsINode* assignedNode = assignedNodes[0];
+
+          aSlot->RemoveAssignedNode(assignedNode);
+          replacementSlot->AppendAssignedNode(assignedNode);
+        }
+      }
+    }
+  }
+}
+
+void
 ShadowRoot::StyleSheetChanged()
 {
   mProtoBinding->FlushSkinSheets();
@@ -222,6 +305,128 @@ ShadowRoot::GetElementsByClassName(const nsAString& aClasses)
   return nsContentUtils::GetElementsByClassName(this, aClasses);
 }
 
+nsresult
+ShadowRoot::GetEventTargetParent(EventChainPreVisitor& aVisitor)
+{
+  aVisitor.mCanHandle = true;
+
+  // https://dom.spec.whatwg.org/#ref-for-get-the-parent%E2%91%A6
+  if (!aVisitor.mEvent->mFlags.mComposed) {
+    nsCOMPtr<nsIContent> originalTarget =
+      do_QueryInterface(aVisitor.mEvent->mOriginalTarget);
+    if (originalTarget->GetContainingShadow() == this) {
+      // If we do stop propagation, we still want to propagate
+      // the event to chrome (nsPIDOMWindow::GetParentTarget()).
+      // The load event is special in that we don't ever propagate it
+      // to chrome.
+      nsCOMPtr<nsPIDOMWindowOuter> win = OwnerDoc()->GetWindow();
+      EventTarget* parentTarget = win && aVisitor.mEvent->mMessage != eLoad
+        ? win->GetParentTarget() : nullptr;
+
+      aVisitor.mParentTarget = parentTarget;
+      return NS_OK;
+    }
+  }
+
+  nsIContent* shadowHost = GetHost();
+  aVisitor.mParentTarget = shadowHost;
+
+  if (aVisitor.mOriginalTargetIsInAnon) {
+    nsCOMPtr<nsIContent> content(do_QueryInterface(aVisitor.mEvent->mTarget));
+    if (content && content->GetBindingParent() == shadowHost) {
+      aVisitor.mEventTargetAtParent = shadowHost;
+    }
+ }
+
+  return NS_OK;
+}
+
+const HTMLSlotElement*
+ShadowRoot::AssignSlotFor(nsIContent* aContent)
+{
+  nsAutoString slotName;
+  // Note that if slot attribute is missing, assign it to the first default
+  // slot, if exists.
+  aContent->GetAttr(kNameSpaceID_None, nsGkAtoms::slot, slotName);
+  nsTArray<HTMLSlotElement*>* slots = mSlotMap.Get(slotName);
+  if (!slots) {
+    return nullptr;
+  }
+
+  HTMLSlotElement* slot = slots->ElementAt(0);
+  MOZ_ASSERT(slot);
+
+  // Find the appropriate position in the assigned node list for the
+  // newly assigned content.
+  const nsTArray<RefPtr<nsINode>>& assignedNodes = slot->AssignedNodes();
+  nsIContent* currentContent = GetHost()->GetFirstChild();
+  bool indexFound = false;
+  uint32_t insertionIndex;
+  for (uint32_t i = 0; i < assignedNodes.Length(); i++) {
+    // Seek through the host's explicit children until the
+    // assigned content is found.
+    while (currentContent && currentContent != assignedNodes[i]) {
+      if (currentContent == aContent) {
+        indexFound = true;
+        insertionIndex = i;
+      }
+
+      currentContent = currentContent->GetNextSibling();
+    }
+
+    if (indexFound) {
+      break;
+    }
+  }
+
+  if (indexFound) {
+    slot->InsertAssignedNode(insertionIndex, aContent);
+  } else {
+    slot->AppendAssignedNode(aContent);
+  }
+
+  return slot;
+}
+
+const HTMLSlotElement*
+ShadowRoot::UnassignSlotFor(nsIContent* aNode, const nsAString& aSlotName)
+{
+  // Find the insertion point to which the content belongs. Note that if slot
+  // attribute is missing, unassign it from the first default slot, if exists.
+  nsTArray<HTMLSlotElement*>* slots = mSlotMap.Get(aSlotName);
+  if (!slots) {
+    return nullptr;
+  }
+
+  HTMLSlotElement* slot = slots->ElementAt(0);
+  MOZ_ASSERT(slot);
+
+  if (!slot->AssignedNodes().Contains(aNode)) {
+    return nullptr;
+  }
+
+  slot->RemoveAssignedNode(aNode);
+  return slot;
+}
+
+bool
+ShadowRoot::MaybeReassignElement(Element* aElement,
+                                 const nsAttrValue* aOldValue)
+{
+  nsIContent* parent = aElement->GetParent();
+  if (parent && parent == GetHost()) {
+    const HTMLSlotElement* oldSlot = UnassignSlotFor(aElement,
+      aOldValue ? aOldValue->GetStringValue() : EmptyString());
+    const HTMLSlotElement* newSlot = AssignSlotFor(aElement);
+
+    if (oldSlot != newSlot) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
 void
 ShadowRoot::DistributionChanged()
 {
@@ -333,7 +538,12 @@ ShadowRoot::AttributeChanged(nsIDocument* aDocument,
                              int32_t aModType,
                              const nsAttrValue* aOldValue)
 {
-  if (!IsPooledNode(aElement)) {
+  if (aNameSpaceID != kNameSpaceID_None || aAttribute != nsGkAtoms::slot) {
+    return;
+  }
+
+  // Attributes may change insertion point matching, find its new distribution.
+  if (!MaybeReassignElement(aElement, aOldValue)) {
     return;
   }
 
@@ -369,11 +579,18 @@ ShadowRoot::ContentInserted(nsIDocument* aDocument,
                             nsIContent* aChild,
                             int32_t aIndexInContainer)
 {
-  if (mInsertionPointChanged) {
-    DistributeAllNodes();
-    mInsertionPointChanged = false;
+  // Check to ensure that the content is in the same anonymous tree
+  // as the container because anonymous content may report its container
+  // as the host but it may not be in the host's child list.
+  if (!nsContentUtils::IsInSameAnonymousTree(aContainer, aChild)) {
     return;
   }
+
+  if (!aChild->IsSlotable() || aContainer != GetHost()) {
+    return;
+  }
+
+  AssignSlotFor(aChild);
 }
 
 void
@@ -383,11 +600,20 @@ ShadowRoot::ContentRemoved(nsIDocument* aDocument,
                            int32_t aIndexInContainer,
                            nsIContent* aPreviousSibling)
 {
-  if (mInsertionPointChanged) {
-    DistributeAllNodes();
-    mInsertionPointChanged = false;
+  // Check to ensure that the content is in the same anonymous tree
+  // as the container because anonymous content may report its container
+  // as the host but it may not be in the host's child list.
+ if (!nsContentUtils::IsInSameAnonymousTree(aContainer, aChild)) {
     return;
   }
+
+  if (!aChild->IsSlotable() || aContainer != GetHost()) {
+    return;
+  }
+
+  nsAutoString slotName;
+  aChild->GetAttr(kNameSpaceID_None, nsGkAtoms::slot, slotName);
+  UnassignSlotFor(aChild, slotName);
 }
 
 nsresult
diff --git a/dom/base/ShadowRoot.h b/dom/base/ShadowRoot.h
index 5efff5be7..c525ba8e8 100644
--- a/dom/base/ShadowRoot.h
+++ b/dom/base/ShadowRoot.h
@@ -21,6 +21,9 @@ class nsIContent;
 class nsXBLPrototypeBinding;
 
 namespace mozilla {
+
+class EventChainPreVisitor;
+
 namespace dom {
 
 class Element;
@@ -73,10 +76,26 @@ public:
 
 private:
   /**
-   * Redistributes a node of the pool, and returns whether the distribution
+   * Try to reassign an element to a slot and returns whether the assignment
    * changed.
    */
-  bool RedistributeElement(Element*);
+  bool MaybeReassignElement(Element* aElement, const nsAttrValue* aOldValue);
+
+  /**
+   * Try to assign aContent to a slot in the shadow tree, returns the assigned
+   * slot if found.
+   */
+  const HTMLSlotElement* AssignSlotFor(nsIContent* aContent);
+
+  /**
+   * Unassign aContent from the assigned slot in the shadow tree, returns the
+   * assigned slot if found.
+   *
+   * Note: slot attribute of aContent may have changed already, so pass slot
+   *       name explicity here.
+   */
+  const HTMLSlotElement* UnassignSlotFor(nsIContent* aContent,
+                                         const nsAString& aSlotName);
 
   /**
    * Called when we redistribute content after insertion points have changed.
@@ -86,6 +105,9 @@ private:
   bool IsPooledNode(nsIContent* aChild) const;
 
 public:
+  void AddSlot(HTMLSlotElement* aSlot);
+  void RemoveSlot(HTMLSlotElement* aSlot);
+
   void SetInsertionPointChanged() { mInsertionPointChanged = true; }
 
   void SetAssociatedBinding(nsXBLBinding* aBinding) { mAssociatedBinding = aBinding; }
@@ -113,11 +135,18 @@ public:
     mIsComposedDocParticipant = aIsComposedDocParticipant;
   }
 
+  nsresult GetEventTargetParent(EventChainPreVisitor& aVisitor) override;
+
 protected:
   virtual ~ShadowRoot();
 
   ShadowRootMode mMode;
 
+  // Map from name of slot to an array of all slots in the shadow DOM with with
+  // the given name. The slots are stored as a weak pointer because the elements
+  // are in the shadow tree and should be kept alive by its parent.
+  nsClassHashtable<nsStringHashKey, nsTArray<mozilla::dom::HTMLSlotElement*>> mSlotMap;
+
   nsTHashtable<nsIdentifierMapEntry> mIdentifierMap;
   nsXBLPrototypeBinding* mProtoBinding;
 
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 038da24b4..62be71b4a 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -44,6 +44,7 @@
 #include "mozilla/dom/Element.h"
 #include "mozilla/dom/FileSystemSecurity.h"
 #include "mozilla/dom/HTMLMediaElement.h"
+#include "mozilla/dom/HTMLSlotElement.h"
 #include "mozilla/dom/HTMLTemplateElement.h"
 #include "mozilla/dom/ipc/BlobChild.h"
 #include "mozilla/dom/ipc/BlobParent.h"
@@ -7053,6 +7054,13 @@ nsContentUtils::HasDistributedChildren(nsIContent* aContent)
     return true;
   }
 
+  HTMLSlotElement* slotEl = HTMLSlotElement::FromContent(aContent);
+  if (slotEl && slotEl->GetContainingShadow()) {
+    // Children of a slot are rendered if the slot does not have any assigned
+    // nodes (fallback content).
+    return slotEl->AssignedNodes().IsEmpty();
+  }
+
   return false;
 }
 
diff --git a/dom/base/nsIContent.h b/dom/base/nsIContent.h
index 8da0ba7f2..975173656 100644
--- a/dom/base/nsIContent.h
+++ b/dom/base/nsIContent.h
@@ -999,6 +999,12 @@ protected:
    */
   nsIAtom* DoGetID() const;
 
+  /**
+   * Returns the assigned slot, if it exists, or the direct parent, if it's a
+   * fallback content of a slot.
+   */
+  nsINode* GetFlattenedTreeParentForMaybeAssignedNode() const;
+
 public:
 #ifdef DEBUG
   /**
diff --git a/dom/base/nsINode.h b/dom/base/nsINode.h
index 7c3eb9134..a07bcae51 100644
--- a/dom/base/nsINode.h
+++ b/dom/base/nsINode.h
@@ -408,6 +408,12 @@ public:
    */
   virtual bool IsNodeOfType(uint32_t aFlags) const = 0;
 
+  bool
+  IsSlotable() const
+  {
+    return IsElement() || IsNodeOfType(eTEXT);
+  }
+
   virtual JSObject* WrapObject(JSContext *aCx, JS::Handle<JSObject*> aGivenProto) override;
 
   /**
diff --git a/dom/events/EventDispatcher.h b/dom/events/EventDispatcher.h
index db7b47dbf..618c863d5 100644
--- a/dom/events/EventDispatcher.h
+++ b/dom/events/EventDispatcher.h
@@ -204,13 +204,6 @@ public:
    * which should be used when the event is handled at mParentTarget.
    */
   dom::EventTarget* mEventTargetAtParent;
-
-  /**
-   * An array of destination insertion points that need to be inserted
-   * into the event path of nodes that are distributed by the
-   * web components distribution algorithm.
-   */
-  nsTArray<nsIContent*> mDestInsertionPoints;
 };
 
 class EventChainPostVisitor : public mozilla::EventChainVisitor
diff --git a/dom/html/HTMLSlotElement.cpp b/dom/html/HTMLSlotElement.cpp
index 9f24f8ba3..1ffde7274 100644
--- a/dom/html/HTMLSlotElement.cpp
+++ b/dom/html/HTMLSlotElement.cpp
@@ -7,6 +7,7 @@
 #include "mozilla/dom/HTMLSlotElement.h"
 #include "mozilla/dom/HTMLSlotElementBinding.h"
 #include "mozilla/dom/HTMLUnknownElement.h"
+#include "mozilla/dom/ShadowRoot.h"
 #include "nsGkAtoms.h"
 #include "nsDocument.h"
 
@@ -15,12 +16,10 @@ NS_NewHTMLSlotElement(already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
                       mozilla::dom::FromParser aFromParser)
 {
   RefPtr<mozilla::dom::NodeInfo> nodeInfo(aNodeInfo);
-  /* Disabled for now
   if (nsDocument::IsWebComponentsEnabled(nodeInfo)) {
     already_AddRefed<mozilla::dom::NodeInfo> nodeInfoArg(nodeInfo.forget());
     return new mozilla::dom::HTMLSlotElement(nodeInfoArg);
   }
-  */
 
   already_AddRefed<mozilla::dom::NodeInfo> nodeInfoArg(nodeInfo.forget());
   return new mozilla::dom::HTMLUnknownElement(nodeInfoArg);
@@ -50,13 +49,161 @@ NS_INTERFACE_MAP_END_INHERITING(nsGenericHTMLElement)
 
 NS_IMPL_ELEMENT_CLONE(HTMLSlotElement)
 
+nsresult
+HTMLSlotElement::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) {
+    containingShadow->AddSlot(this);
+  }
+
+  return NS_OK;
+}
+
+void
+HTMLSlotElement::UnbindFromTree(bool aDeep, bool aNullParent)
+{
+  RefPtr<ShadowRoot> oldContainingShadow = GetContainingShadow();
+
+  nsGenericHTMLElement::UnbindFromTree(aDeep, aNullParent);
+
+  if (oldContainingShadow && !GetContainingShadow()) {
+    oldContainingShadow->RemoveSlot(this);
+  }
+}
+
+nsresult
+HTMLSlotElement::BeforeSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
+                               const nsAttrValueOrString* aValue,
+                               bool aNotify)
+{
+  if (aNameSpaceID == kNameSpaceID_None && aName == nsGkAtoms::name) {
+    if (ShadowRoot* containingShadow = GetContainingShadow()) {
+      containingShadow->RemoveSlot(this);
+    }
+  }
+
+  return nsGenericHTMLElement::BeforeSetAttr(aNameSpaceID, aName, aValue,
+                                             aNotify);
+}
+
+nsresult
+HTMLSlotElement::AfterSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
+                              const nsAttrValue* aValue,
+                              const nsAttrValue* aOldValue,
+                              bool aNotify)
+{
+
+  if (aNameSpaceID == kNameSpaceID_None && aName == nsGkAtoms::name) {
+    if (ShadowRoot* containingShadow = GetContainingShadow()) {
+      containingShadow->AddSlot(this);
+    }
+  }
+
+  return nsGenericHTMLElement::AfterSetAttr(aNameSpaceID, aName, aValue,
+                                            aOldValue, aNotify);
+}
+
+/**
+ * Flatten assigned nodes given a slot, as in:
+ * https://dom.spec.whatwg.org/#find-flattened-slotables
+ */
+static void
+FlattenAssignedNodes(HTMLSlotElement* aSlot, nsTArray<RefPtr<nsINode>>& aNodes)
+{
+  if (!aSlot->GetContainingShadow()) {
+    return;
+  }
+
+  const nsTArray<RefPtr<nsINode>>& assignedNodes = aSlot->AssignedNodes();
+
+  // If assignedNodes is empty, use children of slot as fallback content.
+  if (assignedNodes.IsEmpty()) {
+    for (nsIContent* child = aSlot->AsContent()->GetFirstChild();
+         child;
+         child = child->GetNextSibling()) {
+      if (!child->IsSlotable()) {
+        continue;
+      }
+
+      if (child->IsHTMLElement(nsGkAtoms::slot)) {
+        FlattenAssignedNodes(HTMLSlotElement::FromContent(child), aNodes);
+      } else {
+        aNodes.AppendElement(child);
+      }
+    }
+    return;
+  }
+
+  for (uint32_t i = 0; i < assignedNodes.Length(); i++) {
+    nsINode* assignedNode = assignedNodes[i];
+    if (assignedNode->IsHTMLElement(nsGkAtoms::slot)) {
+      FlattenAssignedNodes(
+        HTMLSlotElement::FromContent(assignedNode->AsContent()), aNodes);
+    } else {
+      aNodes.AppendElement(assignedNode);
+    }
+  }
+}
+
 void
 HTMLSlotElement::AssignedNodes(const AssignedNodesOptions& aOptions,
                                nsTArray<RefPtr<nsINode>>& aNodes)
 {
+  if (aOptions.mFlatten) {
+    return FlattenAssignedNodes(this, aNodes);
+  }
+
   aNodes = mAssignedNodes;
 }
 
+const nsTArray<RefPtr<nsINode>>&
+HTMLSlotElement::AssignedNodes() const
+{
+  return mAssignedNodes;
+}
+
+void
+HTMLSlotElement::InsertAssignedNode(uint32_t aIndex, nsINode* aNode)
+{
+  mAssignedNodes.InsertElementAt(aIndex, aNode);
+  aNode->AsContent()->SetAssignedSlot(this);
+}
+
+void
+HTMLSlotElement::AppendAssignedNode(nsINode* aNode)
+{
+  mAssignedNodes.AppendElement(aNode);
+  aNode->AsContent()->SetAssignedSlot(this);
+}
+
+void
+HTMLSlotElement::RemoveAssignedNode(nsINode* aNode)
+{
+  mAssignedNodes.RemoveElement(aNode);
+  aNode->AsContent()->SetAssignedSlot(nullptr);
+}
+
+void
+HTMLSlotElement::ClearAssignedNodes()
+{
+  for (uint32_t i = 0; i < mAssignedNodes.Length(); i++) {
+    mAssignedNodes[i]->AsContent()->SetAssignedSlot(nullptr);
+  }
+
+  mAssignedNodes.Clear();
+}
+
 JSObject*
 HTMLSlotElement::WrapNode(JSContext* aCx, JS::Handle<JSObject*> aGivenProto)
 {
diff --git a/dom/html/HTMLSlotElement.h b/dom/html/HTMLSlotElement.h
index 187a295db..4f8546200 100644
--- a/dom/html/HTMLSlotElement.h
+++ b/dom/html/HTMLSlotElement.h
@@ -26,6 +26,22 @@ public:
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(HTMLSlotElement, nsGenericHTMLElement)
   virtual nsresult Clone(mozilla::dom::NodeInfo* aNodeInfo, nsINode** aResult) const override;
 
+  // nsIContent
+  virtual nsresult BindToTree(nsIDocument* aDocument, nsIContent* aParent,
+                              nsIContent* aBindingParent,
+                              bool aCompileEventHandlers) override;
+  virtual void UnbindFromTree(bool aDeep = true,
+                              bool aNullParent = true) override;
+
+  virtual nsresult BeforeSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
+                                 const nsAttrValueOrString* aValue,
+                                 bool aNotify) override;
+  virtual nsresult AfterSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
+                                const nsAttrValue* aValue,
+                                const nsAttrValue* aOldValue,
+                                bool aNotify) override;
+
+  // WebIDL
   void SetName(const nsAString& aName, ErrorResult& aRv)
   {
     SetHTMLAttr(nsGkAtoms::name, aName, aRv);
@@ -39,6 +55,13 @@ public:
   void AssignedNodes(const AssignedNodesOptions& aOptions,
                      nsTArray<RefPtr<nsINode>>& aNodes);
 
+  // Helper methods
+  const nsTArray<RefPtr<nsINode>>& AssignedNodes() const;
+  void InsertAssignedNode(uint32_t aIndex, nsINode* aNode);
+  void AppendAssignedNode(nsINode* aNode);
+  void RemoveAssignedNode(nsINode* aNode);
+  void ClearAssignedNodes();
+
 protected:
   virtual ~HTMLSlotElement();
   virtual JSObject*
diff --git a/layout/reftests/webcomponents/basic-insertion-point-1-ref.html b/layout/reftests/webcomponents/basic-insertion-point-1-ref.html
deleted file mode 100644
index 16f6afb28..000000000
--- a/layout/reftests/webcomponents/basic-insertion-point-1-ref.html
+++ /dev/null
@@ -1,8 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<body>
-<div>
-  <div style="border: 10px solid green">Hello</div>
-</div>
-</body>
-</html>
diff --git a/layout/reftests/webcomponents/basic-insertion-point-1.html b/layout/reftests/webcomponents/basic-insertion-point-1.html
deleted file mode 100644
index 727175ef3..000000000
--- a/layout/reftests/webcomponents/basic-insertion-point-1.html
+++ /dev/null
@@ -1,21 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<head>
-  <script>
-    function tweak() {
-      // div with style "border: 10px solid green"
-      var shadowDiv = document.createElement("div");
-      shadowDiv.style.border = "10px solid green";
-
-      var insertionPoint = document.createElement("content");
-      shadowDiv.appendChild(insertionPoint);
-
-      var shadowRoot = document.getElementById('outer').createShadowRoot();
-      shadowRoot.appendChild(shadowDiv);
-    }
-  </script>
-</head>
-<body onload="tweak()">
-<div id="outer">Hello</div>
-</body>
-</html>
diff --git a/layout/reftests/webcomponents/basic-insertion-point-2-ref.html b/layout/reftests/webcomponents/basic-insertion-point-2-ref.html
deleted file mode 100644
index 5e9213775..000000000
--- a/layout/reftests/webcomponents/basic-insertion-point-2-ref.html
+++ /dev/null
@@ -1,11 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<body>
-<div>
-  <div style="border: 10px solid green">
-    <span style="background-color: purple">Hello</span>
-    <span style="background-color: orange">World</span>
-  </div>
-</div>
-</body>
-</html>
diff --git a/layout/reftests/webcomponents/basic-insertion-point-2.html b/layout/reftests/webcomponents/basic-insertion-point-2.html
deleted file mode 100644
index 595edb471..000000000
--- a/layout/reftests/webcomponents/basic-insertion-point-2.html
+++ /dev/null
@@ -1,24 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<head>
-  <script>
-    function tweak() {
-      // div with style "border: 10px solid green"
-      var shadowDiv = document.createElement("div");
-      shadowDiv.style.border = "10px solid green";
-
-      var insertionPoint = document.createElement("content");
-      shadowDiv.appendChild(insertionPoint);
-
-      var shadowRoot = document.getElementById('outer').createShadowRoot();
-      shadowRoot.appendChild(shadowDiv);
-    }
-  </script>
-</head>
-<body onload="tweak()">
-<div id="outer">
-  <span style="background-color: purple">Hello</span>
-  <span style="background-color: orange">World</span>
-</div>
-</body>
-</html>
diff --git a/layout/reftests/webcomponents/basic-shadow-1.html b/layout/reftests/webcomponents/basic-shadow-1.html
index 944a594d2..8949dfc6d 100644
--- a/layout/reftests/webcomponents/basic-shadow-1.html
+++ b/layout/reftests/webcomponents/basic-shadow-1.html
@@ -9,7 +9,7 @@
       shadowDiv.style.height = "100px";
       shadowDiv.style.backgroundColor = "green";
 
-      var shadowRoot = document.getElementById('outer').createShadowRoot();
+      var shadowRoot = document.getElementById('outer').attachShadow({mode: 'open'});
       shadowRoot.appendChild(shadowDiv);
     }
   </script>
diff --git a/layout/reftests/webcomponents/basic-shadow-2.html b/layout/reftests/webcomponents/basic-shadow-2.html
index 587c47221..8e066997d 100644
--- a/layout/reftests/webcomponents/basic-shadow-2.html
+++ b/layout/reftests/webcomponents/basic-shadow-2.html
@@ -6,7 +6,7 @@
       var shadowDiv = document.createElement("div");
       shadowDiv.style.border = "10px solid green";
 
-      var shadowRoot = document.getElementById('outer').createShadowRoot();
+      var shadowRoot = document.getElementById('outer').attachShadow({mode: 'open'});
       shadowRoot.appendChild(shadowDiv);
 
       var orangeDiv = document.createElement("div");
diff --git a/layout/reftests/webcomponents/basic-shadow-3.html b/layout/reftests/webcomponents/basic-shadow-3.html
index 2ae53b5a9..3226c4baa 100644
--- a/layout/reftests/webcomponents/basic-shadow-3.html
+++ b/layout/reftests/webcomponents/basic-shadow-3.html
@@ -6,7 +6,7 @@
       var shadowDiv = document.createElement("div");
       shadowDiv.style.border = "10px solid green";
 
-      var shadowRoot = document.getElementById('outer').createShadowRoot();
+      var shadowRoot = document.getElementById('outer').attachShadow({mode: 'open'});
       shadowRoot.appendChild(shadowDiv);
 
       var orangeDiv = document.createElement("div");
diff --git a/layout/reftests/webcomponents/basic-shadow-4.html b/layout/reftests/webcomponents/basic-shadow-4.html
index 70f91773e..39dc51a9e 100644
--- a/layout/reftests/webcomponents/basic-shadow-4.html
+++ b/layout/reftests/webcomponents/basic-shadow-4.html
@@ -6,7 +6,7 @@
       var shadowDiv = document.createElement("div");
       shadowDiv.style.border = "10px solid green";
 
-      var shadowRoot = document.getElementById('outer').createShadowRoot();
+      var shadowRoot = document.getElementById('outer').attachShadow({mode: 'open'});
       shadowRoot.appendChild(shadowDiv);
 
       var orangeDiv = document.createElement("div");
diff --git a/layout/reftests/webcomponents/basic-slot-1-ref.html b/layout/reftests/webcomponents/basic-slot-1-ref.html
new file mode 100644
index 000000000..4f7418498
--- /dev/null
+++ b/layout/reftests/webcomponents/basic-slot-1-ref.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <style>
+      body { color: green; }
+    </style>
+  </head>
+  <body>
+    This text should be green
+  </body>
+</html>
diff --git a/layout/reftests/webcomponents/basic-slot-1.html b/layout/reftests/webcomponents/basic-slot-1.html
new file mode 100644
index 000000000..b31f4c1c3
--- /dev/null
+++ b/layout/reftests/webcomponents/basic-slot-1.html
@@ -0,0 +1,6 @@
+<!DOCTYPE HTML>
+<html>
+  <body>
+    <slot style="color: green">This text should be green</slot>
+  </body>
+</html>
diff --git a/layout/reftests/webcomponents/basic-slot-2-ref.html b/layout/reftests/webcomponents/basic-slot-2-ref.html
new file mode 100644
index 000000000..d4d1b8c06
--- /dev/null
+++ b/layout/reftests/webcomponents/basic-slot-2-ref.html
@@ -0,0 +1,16 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <style>
+    div {
+      width: 100px;
+      height: 100px;
+      background: green;
+    }
+  </style>
+</head>
+<body>
+  <p>There should be a green box below.</p>
+  <div></div>
+</body>
+</html>
diff --git a/layout/reftests/webcomponents/basic-slot-2.html b/layout/reftests/webcomponents/basic-slot-2.html
new file mode 100644
index 000000000..3754ace20
--- /dev/null
+++ b/layout/reftests/webcomponents/basic-slot-2.html
@@ -0,0 +1,7 @@
+<!DOCTYPE HTML>
+<html>
+  <body>
+    <p>There should be a green box below.</p>
+    <slot style="display: block; width: 100px; height: 100px; background: green;"></slot>
+  </body>
+</html>
diff --git a/layout/reftests/webcomponents/basic-slot-3-ref.html b/layout/reftests/webcomponents/basic-slot-3-ref.html
new file mode 100644
index 000000000..54be54848
--- /dev/null
+++ b/layout/reftests/webcomponents/basic-slot-3-ref.html
@@ -0,0 +1,8 @@
+<!DOCTYPE HTML>
+<html>
+<body>
+<div>
+  <div style="color: green">This text should be green</div>
+</div>
+</body>
+</html>
diff --git a/layout/reftests/webcomponents/basic-slot-3.html b/layout/reftests/webcomponents/basic-slot-3.html
new file mode 100644
index 000000000..c00483fe2
--- /dev/null
+++ b/layout/reftests/webcomponents/basic-slot-3.html
@@ -0,0 +1,18 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <script>
+    function tweak() {
+      var slot = document.createElement("slot");
+      slot.style.color = "green";
+
+      var shadowRoot =
+        document.getElementById('outer').attachShadow({mode: 'open'});
+      shadowRoot.appendChild(slot);
+    }
+  </script>
+</head>
+<body onload="tweak()">
+<div id="outer">This text should be green</div>
+</body>
+</html>
diff --git a/layout/reftests/webcomponents/basic-slot-4.html b/layout/reftests/webcomponents/basic-slot-4.html
new file mode 100644
index 000000000..496a92651
--- /dev/null
+++ b/layout/reftests/webcomponents/basic-slot-4.html
@@ -0,0 +1,20 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <script>
+    function tweak() {
+      var slot = document.createElement("slot");
+      // The border shouldn't be visible, due to display: contents.
+      slot.style.border = "1px solid red";
+      slot.style.color = "green";
+
+      var shadowRoot =
+        document.getElementById('outer').attachShadow({mode: 'open'});
+      shadowRoot.appendChild(slot);
+    }
+  </script>
+</head>
+<body onload="tweak()">
+<div id="outer">This text should be green</div>
+</body>
+</html>
diff --git a/layout/reftests/webcomponents/dynamic-insertion-point-distribution-1.html b/layout/reftests/webcomponents/dynamic-insertion-point-distribution-1.html
index c57f72b37..aefe84f25 100644
--- a/layout/reftests/webcomponents/dynamic-insertion-point-distribution-1.html
+++ b/layout/reftests/webcomponents/dynamic-insertion-point-distribution-1.html
@@ -5,9 +5,11 @@
   <body>
     <div id="host"></div>
     <script>
-      var host = document.getElementById("host");
-      var root = host.createShadowRoot();
-      root.innerHTML = 'a <content></content> c';
+      var host, root;
+
+      host = document.getElementById("host");
+      root = host.attachShadow({mode: 'open'});
+      root.innerHTML = 'a <slot></slot> c';
 
       function tweak() {
         var span = document.createElement("span");
@@ -19,7 +21,7 @@
         document.documentElement.removeAttribute("class");
       }
 
-      window.addEventListener("MozReftestInvalidate", tweak, false);
+      window.addEventListener("MozReftestInvalidate", tweak);
     </script>
   </body>
 </html>
diff --git a/layout/reftests/webcomponents/dynamic-insertion-point-distribution-2.html b/layout/reftests/webcomponents/dynamic-insertion-point-distribution-2.html
index 29a850e90..d753af09c 100644
--- a/layout/reftests/webcomponents/dynamic-insertion-point-distribution-2.html
+++ b/layout/reftests/webcomponents/dynamic-insertion-point-distribution-2.html
@@ -5,8 +5,10 @@
   <body>
     <div id="host"></div>
     <script>
-      var host = document.getElementById("host");
-      var root = host.createShadowRoot();
+      var host, root;
+
+      host = document.getElementById("host");
+      root = host.attachShadow({mode: 'open'});
       root.innerHTML = "<span>a</span>";
 
       function tweak() {
@@ -20,7 +22,7 @@
         document.documentElement.removeAttribute("class");
       }
 
-      window.addEventListener("MozReftestInvalidate", tweak, false);
+      window.addEventListener("MozReftestInvalidate", tweak);
     </script>
   </body>
 </html>
diff --git a/layout/reftests/webcomponents/dynamic-shadow-element-1-ref.html b/layout/reftests/webcomponents/dynamic-shadow-element-1-ref.html
deleted file mode 100644
index a3b5150f6..000000000
--- a/layout/reftests/webcomponents/dynamic-shadow-element-1-ref.html
+++ /dev/null
@@ -1,10 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<head>
-</head>
-<body>
-<div>
-  <div style="background-color: green"><span>Hello</span><span> </span><span>World</span></div>
-</div>
-</body>
-</html>
diff --git a/layout/reftests/webcomponents/dynamic-shadow-element-1.html b/layout/reftests/webcomponents/dynamic-shadow-element-1.html
deleted file mode 100644
index 0142164a1..000000000
--- a/layout/reftests/webcomponents/dynamic-shadow-element-1.html
+++ /dev/null
@@ -1,23 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<head>
-  <script>
-    function tweak() {
-      var oldestShadow = document.getElementById('outer').createShadowRoot();
-      oldestShadow.innerHTML = 'Hello';
-
-      var olderShadow = document.getElementById('outer').createShadowRoot();
-
-      var youngerShadow = document.getElementById('outer').createShadowRoot();
-      youngerShadow.innerHTML = '<div style="background-color:green"><shadow></shadow></div>';
-
-      olderShadow.innerHTML = '<shadow></shadow> World';
-    }
-  </script>
-</head>
-<body onload="tweak()">
-<div id="outer">
-  <div style="width:300px; height:100px; background-color:red;"></div>
-</div>
-</body>
-</html>
diff --git a/layout/reftests/webcomponents/fallback-content-1.html b/layout/reftests/webcomponents/fallback-content-1.html
index ac3b5d02a..0bcd2abbd 100644
--- a/layout/reftests/webcomponents/fallback-content-1.html
+++ b/layout/reftests/webcomponents/fallback-content-1.html
@@ -8,13 +8,13 @@
       shadowDiv.style.border = "10px solid green";
 
       // Insertion point will match nothing and use fallback content.
-      var insertionPoint = document.createElement("content");
-      shadowDiv.appendChild(insertionPoint);
+      var slot = document.createElement("slot");
+      shadowDiv.appendChild(slot);
 
       // Append three nodes as children to use as fallback content.
-      insertionPoint.innerHTML = '<span style="background-color: orange">Hello</span> <span style="background-color: green">World</span>';
+      slot.innerHTML = '<span style="background-color: orange">Hello</span> <span style="background-color: green">World</span>';
 
-      var shadowRoot = document.getElementById('outer').createShadowRoot();
+      var shadowRoot = document.getElementById('outer').attachShadow({mode: 'open'});
       shadowRoot.appendChild(shadowDiv);
     }
   </script>
diff --git a/layout/reftests/webcomponents/input-transition-1.html b/layout/reftests/webcomponents/input-transition-1.html
index ede0fa40b..c11444d05 100644
--- a/layout/reftests/webcomponents/input-transition-1.html
+++ b/layout/reftests/webcomponents/input-transition-1.html
@@ -5,19 +5,21 @@
   <body>
     <div id="host"></div>
     <script>
-      var host = document.getElementById("host");
-      var root = host.createShadowRoot();
+      var host, root;
+
+      host = document.getElementById("host");
+      root = host.attachShadow({mode: 'open'});
       root.innerHTML = '<style>input ~ div { background: red; transition: background 100ms; } input:checked ~ div { background: green; }</style><input id="one" type="checkbox"><div style="height: 50px; width: 50px;"></div>';
 
       function tweak() {
-          var el = root.getElementById("one");
-          el.checked = true;
-          el.nextSibling.addEventListener("transitionend", function() {
-            document.documentElement.removeAttribute("class");
-          }, false);
+        var el = root.getElementById("one");
+        el.checked = true;
+        el.nextSibling.addEventListener("transitionend", function() {
+          setTimeout(()=>{document.documentElement.removeAttribute("class")}, 1000); // wait for the checkbox SVG image to load on Android
+        });
       }
 
-      window.addEventListener("MozReftestInvalidate", tweak, false);
+      window.addEventListener("MozReftestInvalidate", tweak);
     </script>
   </body>
 </html>
diff --git a/layout/reftests/webcomponents/nested-insertion-point-1.html b/layout/reftests/webcomponents/nested-insertion-point-1.html
index 66892029a..3d0d92f0d 100644
--- a/layout/reftests/webcomponents/nested-insertion-point-1.html
+++ b/layout/reftests/webcomponents/nested-insertion-point-1.html
@@ -7,19 +7,20 @@
       var outerShadow = document.createElement("div");
       outerShadow.style.border = "10px solid green";
 
-      var outerInsertionPoint = document.createElement("content");
+      var outerInsertionPoint = document.createElement("slot");
       outerShadow.appendChild(outerInsertionPoint);
 
       // div with style "border: 10px solid orange"
       var innerShadow = document.createElement("div");
       innerShadow.style.border = "10px solid orange";
 
-      var innerInsertionPoint = document.createElement("content");
-      innerShadow.appendChild(innerInsertionPoint);
+      var slot = document.createElement("slot");
+      innerShadow.appendChild(slot);
 
-      outerShadow.createShadowRoot().appendChild(innerShadow);
+      outerShadow.attachShadow({mode: 'open'}).appendChild(innerShadow);
 
-      var shadowRoot = document.getElementById('outer').createShadowRoot();
+      var shadowRoot =
+        document.getElementById('outer').attachShadow({mode: 'open'});
       shadowRoot.appendChild(outerShadow);
     }
   </script>
diff --git a/layout/reftests/webcomponents/reframe-shadow-child-1.html b/layout/reftests/webcomponents/reframe-shadow-child-1.html
new file mode 100644
index 000000000..d953beb6d
--- /dev/null
+++ b/layout/reftests/webcomponents/reframe-shadow-child-1.html
@@ -0,0 +1,16 @@
+<!doctype html>
+<template id="tmpl">
+  <div style="display: table">
+    Some text
+    <span style="display: table-cell">something</span>
+    More text
+  </div>
+</template>
+<div id="host"></div>
+<script>
+  let shadowRoot = document.getElementById("host").attachShadow({mode: 'open'});
+  let tmpl = document.getElementById("tmpl");
+  shadowRoot.appendChild(document.importNode(tmpl.content, true));
+  document.body.offsetTop;
+  shadowRoot.firstElementChild.querySelector("span").remove();
+</script>
diff --git a/layout/reftests/webcomponents/reframe-shadow-child-2.html b/layout/reftests/webcomponents/reframe-shadow-child-2.html
new file mode 100644
index 000000000..0ebbe7433
--- /dev/null
+++ b/layout/reftests/webcomponents/reframe-shadow-child-2.html
@@ -0,0 +1,15 @@
+<!doctype html>
+<template id="tmpl">
+  <div style="display: block">
+    Some text
+    More text
+  </div>
+</template>
+<div id="host"></div>
+<script>
+  let shadowRoot = document.getElementById("host").attachShadow({mode: 'open'});
+  let tmpl = document.getElementById("tmpl");
+  shadowRoot.appendChild(document.importNode(tmpl.content, true));
+  document.body.offsetTop;
+  shadowRoot.firstElementChild.style.display = "table";
+</script>
diff --git a/layout/reftests/webcomponents/reftest.list b/layout/reftests/webcomponents/reftest.list
index 280cd204e..6afbc2720 100644
--- a/layout/reftests/webcomponents/reftest.list
+++ b/layout/reftests/webcomponents/reftest.list
@@ -3,8 +3,6 @@ pref(dom.webcomponents.enabled,true) == basic-shadow-1.html basic-shadow-1-ref.h
 pref(dom.webcomponents.enabled,true) == basic-shadow-2.html basic-shadow-2-ref.html
 pref(dom.webcomponents.enabled,true) == basic-shadow-3.html basic-shadow-3-ref.html
 pref(dom.webcomponents.enabled,true) == basic-shadow-4.html basic-shadow-4-ref.html
-#bug 1421542 pref(dom.webcomponents.enabled,true) == basic-insertion-point-1.html basic-insertion-point-1-ref.html
-#bug 1421542 pref(dom.webcomponents.enabled,true) == basic-insertion-point-2.html basic-insertion-point-2-ref.html
 pref(dom.webcomponents.enabled,true) == fallback-content-1.html fallback-content-1-ref.html
 pref(dom.webcomponents.enabled,true) == remove-insertion-point-1.html remove-insertion-point-1-ref.html
 #bug 1421542 pref(dom.webcomponents.enabled,true) == nested-insertion-point-1.html nested-insertion-point-1-ref.html
@@ -13,3 +11,9 @@ pref(dom.webcomponents.enabled,true) == input-transition-1.html input-transition
 #bug 1421542 pref(dom.webcomponents.enabled,true) == dynamic-insertion-point-distribution-1.html dynamic-insertion-point-distribution-1-ref.html
 pref(dom.webcomponents.enabled,true) == dynamic-insertion-point-distribution-2.html dynamic-insertion-point-distribution-2-ref.html
 pref(dom.webcomponents.enabled,true) == remove-append-shadow-host-1.html remove-append-shadow-host-1-ref.html
+pref(dom.webcomponents.enabled,true) == basic-slot-1.html basic-slot-1-ref.html
+pref(dom.webcomponents.enabled,true) == basic-slot-2.html basic-slot-2-ref.html
+pref(dom.webcomponents.enabled,true) == basic-slot-3.html basic-slot-3-ref.html
+pref(dom.webcomponents.enabled,true) == basic-slot-4.html basic-slot-3-ref.html
+pref(dom.webcomponents.enabled,true) == basic-slot-5.html basic-slot-5-ref.html
+pref(dom.webcomponents.enabled,true) == basic-slot-6.html basic-slot-6-ref.html
diff --git a/layout/reftests/webcomponents/remove-append-shadow-host-1.html b/layout/reftests/webcomponents/remove-append-shadow-host-1.html
index 85161565e..1a752eba6 100644
--- a/layout/reftests/webcomponents/remove-append-shadow-host-1.html
+++ b/layout/reftests/webcomponents/remove-append-shadow-host-1.html
@@ -6,7 +6,7 @@
     <div id="container"><div id="host"></div></div>
     <script>
       var host = document.getElementById("host");
-      var root = host.createShadowRoot();
+      var root = host.attachShadow({mode: 'open'});
       root.innerHTML = 'inside shadow DOM';
 
       var container = document.getElementById("container");
diff --git a/layout/reftests/webcomponents/remove-insertion-point-1.html b/layout/reftests/webcomponents/remove-insertion-point-1.html
index 195673ca8..1b7588daf 100644
--- a/layout/reftests/webcomponents/remove-insertion-point-1.html
+++ b/layout/reftests/webcomponents/remove-insertion-point-1.html
@@ -8,15 +8,15 @@
       shadowDiv.style.border = "10px solid green";
 
       // Insertion point will match nothing and use fallback content.
-      var insertionPoint = document.createElement("content");
-      shadowDiv.appendChild(insertionPoint);
+      var slot = document.createElement("slot");
+      shadowDiv.appendChild(slot);
 
-      var shadowRoot = document.getElementById('outer').createShadowRoot();
+      var shadowRoot = document.getElementById('outer').attachShadow({mode: 'open'});
       shadowRoot.appendChild(shadowDiv);
 
       // Remove the insertion point from the ShadowRoot, "Hello" should no
       // longer be rendered.
-      shadowDiv.removeChild(insertionPoint);
+      shadowDiv.removeChild(slot);
     }
   </script>
 </head>
diff --git a/layout/reftests/webcomponents/style-sharing-across-shadow.html b/layout/reftests/webcomponents/style-sharing-across-shadow.html
new file mode 100644
index 000000000..b41cf7479
--- /dev/null
+++ b/layout/reftests/webcomponents/style-sharing-across-shadow.html
@@ -0,0 +1,22 @@
+<!doctype html>
+<style>
+  div { display: contents }
+</style>
+<div></div>
+<div></div>
+<script>
+  let divs = document.querySelectorAll('div');
+  divs[0].attachShadow({mode: 'open'}).innerHTML = `
+    <style>
+      * { color: green; }
+    </style>
+    <span>Should be green</span>
+  `;
+  divs[1].attachShadow({mode: 'open'}).innerHTML = `
+    <style>
+      * { color: initial; }
+      [foo] { }
+    </style>
+    <span>Should not be green</span>
+  `;
+</script>
diff --git a/layout/reftests/webcomponents/style-sharing.html b/layout/reftests/webcomponents/style-sharing.html
new file mode 100644
index 000000000..0a1e3c95c
--- /dev/null
+++ b/layout/reftests/webcomponents/style-sharing.html
@@ -0,0 +1,14 @@
+<!doctype html>
+<div id="host"></div>
+<script>
+  let root = host.attachShadow({mode: 'open'});
+  root.innerHTML = `
+    <style>
+      #test {
+        color: green;
+      }
+    </style>
+    <span id="test">Should be green</span>
+    <span id="test2">Should not be green</span>
+  `;
+</script>
diff --git a/layout/reftests/webcomponents/update-dist-node-descendants-1.html b/layout/reftests/webcomponents/update-dist-node-descendants-1.html
index e2dd4ebb7..003c23394 100644
--- a/layout/reftests/webcomponents/update-dist-node-descendants-1.html
+++ b/layout/reftests/webcomponents/update-dist-node-descendants-1.html
@@ -5,17 +5,18 @@
 <body>
 <div id="outer"><span id="distnode">text</span></div>
 <script>
-var shadowRoot = document.getElementById('outer').createShadowRoot();
-shadowRoot.innerHTML = '<div><content></content></div>';
 
-function tweak() {
-  var distNode = document.getElementById("distnode");
-  distNode.textContent = "Hello World";
+  var shadowRoot = document.getElementById('outer').attachShadow({mode: 'open'});
+  shadowRoot.innerHTML = '<div><slot></slot></div>';
 
-  document.documentElement.removeAttribute("class");
-}
+  function tweak() {
+    var distNode = document.getElementById("distnode");
+    distNode.textContent = "Hello World";
 
-window.addEventListener("MozReftestInvalidate", tweak);
+    document.documentElement.removeAttribute("class");
+	}
+
+  window.addEventListener("MozReftestInvalidate", tweak);
 </script>
 </body>
 </html>
diff --git a/layout/style/res/ua.css b/layout/style/res/ua.css
index 931b32eb8..504f5dc57 100644
--- a/layout/style/res/ua.css
+++ b/layout/style/res/ua.css
@@ -471,3 +471,9 @@ div:-moz-native-anonymous.moz-custom-content-container {
   width: 100%;
   height: 100%;
 }
+
+/* Shadow DOM v1
+ * https://drafts.csswg.org/css-scoping/#slots-in-shadow-tree */
+slot {
+  display: contents;
+}
\ No newline at end of file
diff --git a/testing/web-platform/meta/html/dom/interfaces.html.ini b/testing/web-platform/meta/html/dom/interfaces.html.ini
index db6a464d0..d0ebb0c7d 100644
--- a/testing/web-platform/meta/html/dom/interfaces.html.ini
+++ b/testing/web-platform/meta/html/dom/interfaces.html.ini
@@ -1,6 +1,6 @@
 [interfaces.html]
   type: testharness
-  prefs: [dom.forms.inputmode:true, dom.dialog_element.enabled:true]
+  prefs: [dom.forms.inputmode:true, dom.dialog_element.enabled:true, dom.webcomponents.enabled:true]
   [Document interface: attribute domain]
     expected: FAIL
 
@@ -2524,21 +2524,6 @@
   [HTMLSlotElement interface: operation assignedNodes(AssignedNodesOptions)]
     expected: FAIL
 
-  [HTMLSlotElement must be primary interface of document.createElement("slot")]
-    expected: FAIL
-
-  [Stringification of document.createElement("slot")]
-    expected: FAIL
-
-  [HTMLSlotElement interface: document.createElement("slot") must inherit property "name" with the proper type (0)]
-    expected: FAIL
-
-  [HTMLSlotElement interface: document.createElement("slot") must inherit property "assignedNodes" with the proper type (1)]
-    expected: FAIL
-
-  [HTMLSlotElement interface: calling assignedNodes(AssignedNodesOptions) on document.createElement("slot") with too few arguments must throw TypeError]
-    expected: FAIL
-
   [HTMLInputElement interface: createInput("text") must inherit property "dirName" with the proper type (6)]
     expected: FAIL
 
diff --git a/testing/web-platform/meta/html/dom/reflection-misc.html.ini b/testing/web-platform/meta/html/dom/reflection-misc.html.ini
index bc65d4191..05bb9c4cd 100644
--- a/testing/web-platform/meta/html/dom/reflection-misc.html.ini
+++ b/testing/web-platform/meta/html/dom/reflection-misc.html.ini
@@ -1,6 +1,6 @@
 [reflection-misc.html]
   type: testharness
-  prefs: [dom.dialog_element.enabled: true]
+  prefs: [dom.dialog_element.enabled: true, dom.webcomponents.enabled:true]
   [html.tabIndex: setAttribute() to object "3" followed by getAttribute()]
     expected: FAIL
 
diff --git a/testing/web-platform/meta/shadow-dom/HTMLSlotElement-interface.html.ini b/testing/web-platform/meta/shadow-dom/HTMLSlotElement-interface.html.ini
deleted file mode 100644
index c045a3006..000000000
--- a/testing/web-platform/meta/shadow-dom/HTMLSlotElement-interface.html.ini
+++ /dev/null
@@ -1,44 +0,0 @@
-[HTMLSlotElement-interface.html]
-  type: testharness
-  [HTMLSlotElement must be defined on window]
-    expected: FAIL
-
-  ["name" attribute on HTMLSlotElement must reflect "name" attribute]
-    expected: FAIL
-
-  [assignedNodes() on a HTMLSlotElement must return an empty array when the slot element is not in a tree or in a document tree]
-    expected: FAIL
-
-  [assignedNodes({"flattened":false}) on a HTMLSlotElement must return an empty array when the slot element is not in a tree or in a document tree]
-    expected: FAIL
-
-  [assignedNodes({"flattened":true}) on a HTMLSlotElement must return an empty array when the slot element is not in a tree or in a document tree]
-    expected: FAIL
-
-  [assignedNodes() must return the list of assigned nodes when none of the assigned nodes themselves are slots]
-    expected: FAIL
-
-  [assignedNodes({"flattened":false}) must return the list of assigned nodes when none of the assigned nodes themselves are slots]
-    expected: FAIL
-
-  [assignedNodes({"flattened":true}) must return the list of assigned nodes when none of the assigned nodes themselves are slots]
-    expected: FAIL
-
-  [assignedNodes() must update when slot and name attributes are modified]
-    expected: FAIL
-
-  [assignedNodes({"flattened":false}) must update when slot and name attributes are modified]
-    expected: FAIL
-
-  [assignedNodes({"flattened":true}) must update when slot and name attributes are modified]
-    expected: FAIL
-
-  [assignedNodes must update when a default slot is introduced dynamically by a slot rename]
-    expected: FAIL
-
-  [assignedNodes must update when slot elements are inserted or removed]
-    expected: FAIL
-
-  [assignedNodes({flatten: true}) must return the distributed nodes, and assignedNodes() and assignedNodes({flatten: false}) must returned the assigned nodes]
-    expected: FAIL
-
diff --git a/testing/web-platform/meta/shadow-dom/Slotable-interface.html.ini b/testing/web-platform/meta/shadow-dom/Slotable-interface.html.ini
deleted file mode 100644
index 6ef193dee..000000000
--- a/testing/web-platform/meta/shadow-dom/Slotable-interface.html.ini
+++ /dev/null
@@ -1,12 +0,0 @@
-[Slotable-interface.html]
-  type: testharness
-  prefs: [dom.webcomponents.enabled:true]
-  [assignedSlot must return null when the node does not have an assigned node]
-    expected: FAIL
-
-  [assignedSlot must return the assigned slot]
-    expected: FAIL
-
-  [assignedSlot must return null when the assigned slot element is inside a closed shadow tree]
-    expected: FAIL
-
diff --git a/testing/web-platform/meta/shadow-dom/event-inside-slotted-node.html.ini b/testing/web-platform/meta/shadow-dom/event-inside-slotted-node.html.ini
deleted file mode 100644
index 7e42670a3..000000000
--- a/testing/web-platform/meta/shadow-dom/event-inside-slotted-node.html.ini
+++ /dev/null
@@ -1,38 +0,0 @@
-[event-inside-slotted-node.html]
-  type: testharness
-  [Firing an event inside a grand child of a detached open mode shadow host]
-    expected: FAIL
-
-  [Firing an event inside a grand child of a detached closed mode shadow host]
-    expected: FAIL
-
-  [Firing an event inside a grand child of an in-document open mode shadow host]
-    expected: FAIL
-
-  [Firing an event inside a grand child of an in-document closed mode shadow host]
-    expected: FAIL
-
-  [Firing an event on a node with two ancestors with a detached open and open shadow trees with an inner open shadow tree]
-    expected: FAIL
-
-  [Firing an event on a node with two ancestors with a detached open and open shadow trees with an inner closed shadow tree]
-    expected: FAIL
-
-  [Firing an event on a node with two ancestors with a detached open and closed shadow trees with an inner open shadow tree]
-    expected: FAIL
-
-  [Firing an event on a node with two ancestors with a detached open and closed shadow trees with an inner closed shadow tree]
-    expected: FAIL
-
-  [Firing an event on a node with two ancestors with a detached closed and open shadow trees with an inner open shadow tree]
-    expected: FAIL
-
-  [Firing an event on a node with two ancestors with a detached closed and open shadow trees with an inner closed shadow tree]
-    expected: FAIL
-
-  [Firing an event on a node with two ancestors with a detached closed and closed shadow trees with an inner open shadow tree]
-    expected: FAIL
-
-  [Firing an event on a node with two ancestors with a detached closed and closed shadow trees with an inner closed shadow tree]
-    expected: FAIL
-
diff --git a/testing/web-platform/meta/shadow-dom/slots-fallback.html.ini b/testing/web-platform/meta/shadow-dom/slots-fallback.html.ini
deleted file mode 100644
index d32ae812b..000000000
--- a/testing/web-platform/meta/shadow-dom/slots-fallback.html.ini
+++ /dev/null
@@ -1,32 +0,0 @@
-[slots-fallback.html]
-  type: testharness
-  [Slots fallback: Basic.]
-    expected: FAIL
-
-  [Slots fallback: Slots in Slots.]
-    expected: FAIL
-
-  [Slots fallback: Fallback contents should not be used if a node is assigned.]
-    expected: FAIL
-
-  [Slots fallback: Slots in Slots: Assinged nodes should be used as fallback contents of another slot]
-    expected: FAIL
-
-  [Slots fallback: Complex case.]
-    expected: FAIL
-
-  [Slots fallback: Mutation. Append fallback contents.]
-    expected: FAIL
-
-  [Slots fallback: Mutation. Remove fallback contents.]
-    expected: FAIL
-
-  [Slots fallback: Mutation. Assign a node to a slot so that fallback contens are no longer used.]
-    expected: FAIL
-
-  [Slots fallback: Mutation. Remove an assigned node from a slot so that fallback contens will be used.]
-    expected: FAIL
-
-  [Slots fallback: Mutation.  Remove a slot which is a fallback content of another slot.]
-    expected: FAIL
-
diff --git a/testing/web-platform/meta/shadow-dom/slots.html.ini b/testing/web-platform/meta/shadow-dom/slots.html.ini
deleted file mode 100644
index 3c047e482..000000000
--- a/testing/web-platform/meta/shadow-dom/slots.html.ini
+++ /dev/null
@@ -1,71 +0,0 @@
-[slots.html]
-  type: testharness
-  [Slots: Basic.]
-    expected: FAIL
-
-  [Slots: Slots in closed.]
-    expected: FAIL
-
-  [Slots: Slots not in a shadow tree.]
-    expected: FAIL
-
-  [Slots: Distributed nooes for Slots not in a shadow tree.]
-    expected: FAIL
-
-  [Slots: Name matching]
-    expected: FAIL
-
-  [Slots: No direct host child.]
-    expected: FAIL
-
-  [Slots: Default Slot.]
-    expected: FAIL
-
-  [Slots: Slot in Slot does not matter in assignment.]
-    expected: FAIL
-
-  [Slots: Slot is assigned to another slot]
-    expected: FAIL
-
-  [Slots: Open > Closed.]
-    expected: FAIL
-
-  [Slots: Closed > Closed.]
-    expected: FAIL
-
-  [Slots: Closed > Open.]
-    expected: FAIL
-
-  [Slots: Complex case: Basi line.]
-    expected: FAIL
-
-  [Slots: Mutation: appendChild.]
-    expected: FAIL
-
-  [Slots: Mutation: Change slot= attribute 1.]
-    expected: FAIL
-
-  [Slots: Mutation: Change slot= attribute 2.]
-    expected: FAIL
-
-  [Slots: Mutation: Change slot= attribute 3.]
-    expected: FAIL
-
-  [Slots: Mutation: Remove a child.]
-    expected: FAIL
-
-  [Slots: Mutation: Add a slot: after.]
-    expected: FAIL
-
-  [Slots: Mutation: Add a slot: before.]
-    expected: FAIL
-
-  [Slots: Mutation: Remove a slot.]
-    expected: FAIL
-
-  [Slots: Mutation: Change slot name= attribute.]
-    expected: FAIL
-
-  [Slots: Mutation: Change slot slot= attribute.]
-    expected: FAIL
-