--- a/dom/smil/nsSMILCompositor.cpp
+++ b/dom/smil/nsSMILCompositor.cpp
@@ -123,38 +123,50 @@ nsSMILCompositor::ClearAnimationEffects(
   smilAttr->ClearAnimValue();
 }
 
 // Protected Helper Functions
 // --------------------------
 UniquePtr<nsISMILAttr>
 nsSMILCompositor::CreateSMILAttr()
 {
-  nsCSSPropertyID propID =
-    nsCSSProps::LookupProperty(nsDependentAtomString(mKey.mAttributeName),
-                               CSSEnabledState::eForAllContent);
-  if (nsSMILCSSProperty::IsPropertyAnimatable(propID)) {
-    // 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
-    // SVG-namespace element mapping its width/height attribute to style.
-    bool animateAsAttr = (mKey.mAttributeName == nsGkAtoms::width ||
-                          mKey.mAttributeName == nsGkAtoms::height) &&
-                         mKey.mElement->GetNameSpaceID() == kNameSpaceID_SVG &&
-                         !mKey.mElement->IsAttributeMapped(mKey.mAttributeName);
-    if (!animateAsAttr) {
-      return MakeUnique<nsSMILCSSProperty>(propID, mKey.mElement.get());
-    }
+  nsCSSPropertyID propID = GetCSSPropertyToAnimate();
+
+  if (propID != eCSSProperty_UNKNOWN) {
+    return MakeUnique<nsSMILCSSProperty>(propID, mKey.mElement.get());
   }
 
   return mKey.mElement->GetAnimatedAttr(mKey.mAttributeNamespaceID,
                                         mKey.mAttributeName);
 }
 
+nsCSSPropertyID
+nsSMILCompositor::GetCSSPropertyToAnimate() const
+{
+  nsCSSPropertyID propID =
+    nsCSSProps::LookupProperty(nsDependentAtomString(mKey.mAttributeName),
+                               CSSEnabledState::eForAllContent);
+
+  if (!nsSMILCSSProperty::IsPropertyAnimatable(propID)) {
+    return eCSSProperty_UNKNOWN;
+  }
+
+  // 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
+  // SVG-namespace element mapping its width/height attribute to style.
+  bool animateAsAttr = (mKey.mAttributeName == nsGkAtoms::width ||
+                        mKey.mAttributeName == nsGkAtoms::height) &&
+                       mKey.mElement->GetNameSpaceID() == kNameSpaceID_SVG &&
+                       !mKey.mElement->IsAttributeMapped(mKey.mAttributeName);
+
+  return animateAsAttr ? eCSSProperty_UNKNOWN : propID;
+}
+
 uint32_t
 nsSMILCompositor::GetFirstFuncToAffectSandwich()
 {
   // For performance reasons, we throttle most animations on elements in
   // display:none subtrees. (We can't throttle animations that target the
   // "display" property itself, though -- if we did, display:none elements
   // could never be dynamically displayed via animations.)
   // To determine whether we're in a display:none subtree, we will check the

--- a/dom/smil/nsSMILCompositor.h
+++ b/dom/smil/nsSMILCompositor.h
@@ -71,16 +71,20 @@ public:
   void StealCachedBaseValue(nsSMILCompositor* aOther) {
     mCachedBaseValue = mozilla::Move(aOther->mCachedBaseValue);
   }
 
  private:
   // Create a nsISMILAttr for my target, on the heap.
   mozilla::UniquePtr<nsISMILAttr> CreateSMILAttr();
 
+  // Returns the CSS property this compositor should animate, or
+  // eCSSProperty_UNKNOWN if this compositor does not animate a CSS property.
+  nsCSSPropertyID GetCSSPropertyToAnimate() const;
+
   // Finds the index of the first function that will affect our animation
   // sandwich. Also toggles the 'mForceCompositing' flag if it finds that any
   // (used) functions have changed.
   uint32_t GetFirstFuncToAffectSandwich();
 
   // If the passed-in base value differs from our cached base value, this
   // method updates the cached value (and toggles the 'mForceCompositing' flag)
   void UpdateCachedBaseValue(const nsSMILValue& aBaseValue);