diff --git a/layout/style/nsComputedDOMStyle.cpp b/layout/style/nsComputedDOMStyle.cpp
index 68e3b4053b33..26b2cf888eac 100644
--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -264,9 +264,23 @@ nsComputedDOMStyle::nsComputedDOMStyle(dom::Element* aElement,
 
 nsComputedDOMStyle::~nsComputedDOMStyle()
 {
+  ClearStyleContext();
 }
 
-NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(nsComputedDOMStyle, mContent)
+NS_IMPL_CYCLE_COLLECTION_CLASS(nsComputedDOMStyle)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsComputedDOMStyle)
+  tmp->ClearStyleContext();  // remove observer before clearing mContent
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mContent)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(nsComputedDOMStyle)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mContent)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE_SCRIPT_OBJECTS
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_TRACE_WRAPPERCACHE(nsComputedDOMStyle)
 
 NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_BEGIN(nsComputedDOMStyle)
   return tmp->IsBlack();
@@ -283,6 +297,7 @@ NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_THIS_END
 // QueryInterface implementation for nsComputedDOMStyle
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsComputedDOMStyle)
   NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsIMutationObserver)
 NS_INTERFACE_MAP_END_INHERITING(nsDOMCSSDeclaration)
 
 
@@ -571,13 +586,39 @@ nsComputedDOMStyle::GetCSSParsingEnvironment(CSSParsingEnvironment& aCSSParseEnv
   aCSSParseEnv.mPrincipal = nullptr;
 }
 
+void
+nsComputedDOMStyle::ClearStyleContext()
+{
+  if (mResolvedStyleContext) {
+    mResolvedStyleContext = false;
+    mContent->RemoveMutationObserver(this);
+  }
+  mStyleContext = nullptr;
+}
+
+void
+nsComputedDOMStyle::SetResolvedStyleContext(nsRefPtr<nsStyleContext>&& aContext)
+{
+  if (!mResolvedStyleContext) {
+    mResolvedStyleContext = true;
+    mContent->AddMutationObserver(this);
+  }
+  mStyleContext = aContext;
+}
+
+void
+nsComputedDOMStyle::SetFrameStyleContext(nsStyleContext* aContext)
+{
+  ClearStyleContext();
+  mStyleContext = aContext;
+}
+
 void
 nsComputedDOMStyle::UpdateCurrentStyleSources(bool aNeedsLayoutFlush)
 {
   nsCOMPtr<nsIDocument> document = do_QueryReferent(mDocumentWeak);
   if (!document) {
-    mResolvedStyleContext = false;
-    mStyleContext = nullptr;
+    ClearStyleContext();
     return;
   }
 
@@ -595,8 +636,7 @@ nsComputedDOMStyle::UpdateCurrentStyleSources(bool aNeedsLayoutFlush)
 
   mPresShell = document->GetShell();
   if (!mPresShell || !mPresShell->GetPresContext()) {
-    mResolvedStyleContext = false;
-    mStyleContext = nullptr;
+    ClearStyleContext();
     return;
   }
 
@@ -632,8 +672,7 @@ nsComputedDOMStyle::UpdateCurrentStyleSources(bool aNeedsLayoutFlush)
                      "the inner table");
       }
 
-      mStyleContext = mInnerFrame->StyleContext();
-      mResolvedStyleContext = false;
+      SetFrameStyleContext(mInnerFrame->StyleContext());
       NS_ASSERTION(mStyleContext, "Frame without style context?");
     }
   }
@@ -661,13 +700,13 @@ nsComputedDOMStyle::UpdateCurrentStyleSources(bool aNeedsLayoutFlush)
     }
 #endif
     // Need to resolve a style context
-    mStyleContext =
+    nsRefPtr<nsStyleContext> resolvedStyleContext =
       nsComputedDOMStyle::GetStyleContextForElement(mContent->AsElement(),
                                                     mPseudo,
                                                     mPresShell,
                                                     mStyleType);
-    if (!mStyleContext) {
-      mResolvedStyleContext = false;
+    if (!resolvedStyleContext) {
+      ClearStyleContext();
       return;
     }
 
@@ -678,8 +717,7 @@ nsComputedDOMStyle::UpdateCurrentStyleSources(bool aNeedsLayoutFlush)
                    mPresShell->GetPresContext()->GetRestyleGeneration(),
                  "why should we have flushed style again?");
 
-    mResolvedStyleContext = true;
-    mStyleContextGeneration = currentGeneration;
+    SetResolvedStyleContext(Move(resolvedStyleContext));
     NS_ASSERTION(mPseudo || !mStyleContext->HasPseudoElementData(),
                  "should not have pseudo-element data");
   }
@@ -6018,6 +6056,17 @@ nsComputedDOMStyle::DoGetCustomProperty(const nsAString& aPropertyName)
   return val;
 }
 
+void
+nsComputedDOMStyle::ParentChainChanged(nsIContent* aContent)
+{
+  NS_ASSERTION(mContent == aContent, "didn't we register mContent?");
+  NS_ASSERTION(mResolvedStyleContext,
+               "should have only registered an observer when "
+               "mResolvedStyleContext is true");
+
+  ClearStyleContext();
+}
+
 /* static */ nsComputedStyleMap*
 nsComputedDOMStyle::GetComputedStyleMap()
 {
diff --git a/layout/style/nsComputedDOMStyle.h b/layout/style/nsComputedDOMStyle.h
index b8e40cfc8eec..7808cc5e6550 100644
--- a/layout/style/nsComputedDOMStyle.h
+++ b/layout/style/nsComputedDOMStyle.h
@@ -45,6 +45,7 @@ class nsStyleSides;
 struct nsTimingFunction;
 
 class nsComputedDOMStyle final : public nsDOMCSSDeclaration
+                               , public nsStubMutationObserver
 {
 public:
   typedef nsCSSProps::KTableValue KTableValue;
@@ -110,6 +111,9 @@ public:
   static void RegisterPrefChangeCallbacks();
   static void UnregisterPrefChangeCallbacks();
 
+  // nsIMutationObserver
+  NS_DECL_NSIMUTATIONOBSERVER_PARENTCHAINCHANGED
+
 private:
   virtual ~nsComputedDOMStyle();
 
@@ -128,6 +132,11 @@ private:
   void UpdateCurrentStyleSources(bool aNeedsLayoutFlush);
   void ClearCurrentStyleSources();
 
+  // Helper functions called by UpdateCurrentStyleSources.
+  void ClearStyleContext();
+  void SetResolvedStyleContext(nsRefPtr<nsStyleContext>&& aContext);
+  void SetFrameStyleContext(nsStyleContext* aContext);
+
 #define STYLE_STRUCT(name_, checkdata_cb_)                              \
   const nsStyle##name_ * Style##name_() {                               \
     return mStyleContext->Style##name_();                               \