diff --git a/gfx/layers/Layers.h b/gfx/layers/Layers.h
index 56926634c797..61d4a5862747 100644
--- a/gfx/layers/Layers.h
+++ b/gfx/layers/Layers.h
@@ -1108,6 +1108,7 @@ public:
 
   // Call AddAnimation to add a new animation to this layer from layout code.
   // Caller must fill in all the properties of the returned animation.
+  // A later animation overrides an earlier one.
   Animation* AddAnimation();
   // ClearAnimations clears animations on this layer.
   void ClearAnimations();
diff --git a/gfx/layers/composite/AsyncCompositionManager.cpp b/gfx/layers/composite/AsyncCompositionManager.cpp
index 5cc6a4c008de..712b17b10366 100644
--- a/gfx/layers/composite/AsyncCompositionManager.cpp
+++ b/gfx/layers/composite/AsyncCompositionManager.cpp
@@ -440,7 +440,8 @@ SampleAnimations(Layer* aLayer, TimeStamp aPoint)
 
   bool activeAnimations = false;
 
-  for (uint32_t i = animations.Length(); i-- !=0; ) {
+  // Process in order, since later animations override earlier ones.
+  for (size_t i = 0, iEnd = animations.Length(); i < iEnd; ++i) {
     Animation& animation = animations[i];
     AnimData& animData = animationData[i];
 
diff --git a/layout/base/nsDisplayList.cpp b/layout/base/nsDisplayList.cpp
index f42895ab6d0d..8dcc4def286b 100644
--- a/layout/base/nsDisplayList.cpp
+++ b/layout/base/nsDisplayList.cpp
@@ -413,6 +413,7 @@ AddAnimationsForProperty(nsIFrame* aFrame, nsCSSProperty aProperty,
                                       CSS_PROPERTY_CAN_ANIMATE_ON_COMPOSITOR),
              "inconsistent property flags");
 
+  // Add from first to last (since last overrides)
   for (size_t playerIdx = 0; playerIdx < aPlayers.Length(); playerIdx++) {
     AnimationPlayer* player = aPlayers[playerIdx];
     if (!player->IsPlaying()) {
@@ -560,6 +561,8 @@ nsDisplayListBuilder::AddAnimationsAndTransitionsToLayer(Layer* aLayer,
     data = null_t();
   }
 
+  // When both are running, animations override transitions.  We want
+  // to add the ones that override last.
   if (transitions) {
     AddAnimationsForProperty(aFrame, aProperty, transitions->mPlayers,
                              aLayer, data, pending);