--- a/dom/smil/nsSMILCompositor.cpp
+++ b/dom/smil/nsSMILCompositor.cpp
@@ -119,23 +119,28 @@ nsSMILCompositor::ClearAnimationEffects(
   if (!smilAttr) {
     // Target attribute not found (or, out of memory)
     return;
   }
   smilAttr->ClearAnimValue();
 }
 
 /*static*/ bool
-nsSMILCompositor::IsCSSPropertyAnimation(nsIAtom* aAttributeName,
+nsSMILCompositor::IsCSSPropertyAnimation(int32_t aAttributeNamespaceID,
+                                         nsIAtom* aAttributeName,
                                          nsCSSPropertyID aPropID,
                                          mozilla::dom::Element* aTargetElement)
 {
   MOZ_ASSERT(aAttributeName);
   MOZ_ASSERT(aTargetElement);
 
+  if (aAttributeNamespaceID != kNameSpaceID_None) {
+    return false;
+  }
+
   if (!nsSMILCSSProperty::IsPropertyAnimatable(aPropID)) {
     return false;
   }
 
   // If we are animating the 'width' or 'height' of an outer SVG
   // element we should animate it as a CSS property, but for other elements
   // (e.g. <rect>) we should animate it as a length attribute.
   // The easiest way to test for an outer SVG element, is to see if it is an
@@ -152,17 +157,18 @@ nsSMILCompositor::IsCSSPropertyAnimation
 // --------------------------
 UniquePtr<nsISMILAttr>
 nsSMILCompositor::CreateSMILAttr()
 {
   nsCSSPropertyID propID =
     nsCSSProps::LookupProperty(nsDependentAtomString(mKey.mAttributeName),
                                CSSEnabledState::eForAllContent);
 
-  if (IsCSSPropertyAnimation(mKey.mAttributeName, propID, mKey.mElement)) {
+  if (IsCSSPropertyAnimation(mKey.mAttributeNamespaceID, mKey.mAttributeName,
+                             propID, mKey.mElement)) {
     return MakeUnique<nsSMILCSSProperty>(propID, mKey.mElement.get());
   }
 
   return mKey.mElement->GetAnimatedAttr(mKey.mAttributeNamespaceID,
                                         mKey.mAttributeName);
 }
 
 uint32_t

--- a/dom/smil/nsSMILCompositor.h
+++ b/dom/smil/nsSMILCompositor.h
@@ -67,24 +67,25 @@ public:
   // optimizations) when we hit ComposeAttribute.
   void ToggleForceCompositing() { mForceCompositing = true; }
 
   // Transfers |aOther|'s mCachedBaseValue to |this|
   void StealCachedBaseValue(nsSMILCompositor* aOther) {
     mCachedBaseValue = mozilla::Move(aOther->mCachedBaseValue);
   }
 
-  // Returns true if, given an attributeName and corresponding property ID,
-  // an animation targetting |aTargetElement| will animate the corresponding CSS
-  // property rather than an attribute.
+  // Returns true if, given an attributeName in a specified namespace and
+  // its corresponding property ID, an animation targetting |aTargetElement|
+  // will animate the corresponding CSS property rather than an attribute.
   //
   // (The property ID could be derived from |aAttributeName| but is passed as
   // a separate argument here as an optimization since call sites of this
   // method may require |aPropID| beyond the scope of this function.)
-  static bool IsCSSPropertyAnimation(nsIAtom* aAttributeName,
+  static bool IsCSSPropertyAnimation(int32_t aAttributeNamespaceID,
+                                     nsIAtom* aAttributeName,
                                      nsCSSPropertyID aPropID,
                                      mozilla::dom::Element* aTargetElement);
 
  private:
   // Create a nsISMILAttr for my target, on the heap.
   UniquePtr<nsISMILAttr> CreateSMILAttr();
 
   // Finds the index of the first function that will affect our animation