--- a/layout/base/RestyleManager.cpp
+++ b/layout/base/RestyleManager.cpp
@@ -61,16 +61,17 @@ FrameTagToString(const nsIFrame* aFrame)
 
 RestyleManager::RestyleManager(nsPresContext* aPresContext)
   : mPresContext(aPresContext)
   , mRebuildAllStyleData(false)
   , mObservingRefreshDriver(false)
   , mInStyleRefresh(false)
   , mSkipAnimationRules(false)
   , mPostAnimationRestyles(false)
+  , mIsProcessingAnimationStyleChange(false)
   , mHoverGeneration(0)
   , mRebuildAllExtraHint(nsChangeHint(0))
   , mLastUpdateForThrottledAnimations(aPresContext->RefreshDriver()->
                                         MostRecentRefresh())
   , mAnimationGeneration(0)
   , mReframingStyleContexts(nullptr)
   , mPendingRestyles(ELEMENT_HAS_PENDING_RESTYLE |
                      ELEMENT_IS_POTENTIAL_RESTYLE_ROOT)
@@ -1570,17 +1571,21 @@ RestyleManager::ProcessPendingRestyles()
   // second because we need to start animations that resulted from the
   // first set of restyles (e.g., CSS transitions with negative
   // transition-delay), and because we need to immediately
   // restyle-with-animation any just-restyled elements that are
   // mid-transition (since processing the non-animation restyle ignores
   // the running transition so it can check for a new change on the same
   // property, and then posts an immediate animation style change).
   mPresContext->SetProcessingAnimationStyleChange(true);
+  MOZ_ASSERT(!mIsProcessingAnimationStyleChange, "nesting forbidden");
+  mIsProcessingAnimationStyleChange = true;
   mPendingAnimationRestyles.ProcessRestyles();
+  MOZ_ASSERT(mIsProcessingAnimationStyleChange, "nesting forbidden");
+  mIsProcessingAnimationStyleChange = false;
   mPresContext->SetProcessingAnimationStyleChange(false);
 
   mPresContext->SetProcessingRestyles(false);
 #ifdef DEBUG
   mIsProcessingRestyles = false;
 #endif
   NS_POSTCONDITION(mPendingRestyles.Count() == oldPendingRestyleCount,
                    "We should not have posted new non-animation restyles while "

--- a/layout/base/RestyleManager.h
+++ b/layout/base/RestyleManager.h
@@ -103,16 +103,22 @@ public:
   // styles associated with animation.  Only true when
   // SkipAnimationRules() is also true.
   bool PostAnimationRestyles() const {
     MOZ_ASSERT(mSkipAnimationRules || !mPostAnimationRestyles,
                "inconsistent state");
     return mPostAnimationRestyles;
   }
 
+  // Whether we're currently in the animation phase of restyle
+  // processing (to be eliminated in bug 960465)
+  bool IsProcessingAnimationStyleChange() const {
+    return mIsProcessingAnimationStyleChange;
+  }
+
   /**
    * Reparent the style contexts of this frame subtree.  The parent frame of
    * aFrame must be changed to the new parent before this function is called;
    * the new parent style context will be automatically computed based on the
    * new position in the frame tree.
    *
    * @param aFrame the root of the subtree to reparent.  Must not be null.
    */
@@ -444,16 +450,19 @@ private:
   // True if we're in the middle of a nsRefreshDriver refresh
   bool mInStyleRefresh : 1;
   // Whether rule matching should skip styles associated with animation
   bool mSkipAnimationRules : 1;
   // Whether rule matching should post animation restyles when it skips
   // styles associated with animation.  Only true when
   // mSkipAnimationRules is also true.
   bool mPostAnimationRestyles : 1;
+  // Whether we're currently in the animation phase of restyle
+  // processing (to be eliminated in bug 960465)
+  bool mIsProcessingAnimationStyleChange : 1;
 
   uint32_t mHoverGeneration;
   nsChangeHint mRebuildAllExtraHint;
 
   mozilla::TimeStamp mLastUpdateForThrottledAnimations;
 
   OverflowChangedTracker mOverflowChangedTracker;