--- a/dom/smil/nsSMILCompositor.cpp
+++ b/dom/smil/nsSMILCompositor.cpp
@@ -118,37 +118,52 @@ nsSMILCompositor::ClearAnimationEffects(
   UniquePtr<nsISMILAttr> smilAttr{CreateSMILAttr()};
   if (!smilAttr) {
     // Target attribute not found (or, out of memory)
     return;
   }
   smilAttr->ClearAnimValue();
 }
 
+/*static*/ bool
+nsSMILCompositor::IsCSSPropertyAnimation(nsIAtom* aAttributeName,
+                                         nsCSSPropertyID aPropID,
+                                         mozilla::dom::Element* aTargetElement)
+{
+  MOZ_ASSERT(aAttributeName);
+  MOZ_ASSERT(aTargetElement);
+
+  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
+  // SVG-namespace element mapping its width/height attribute to style.
+  bool animateAsAttr = (aAttributeName == nsGkAtoms::width ||
+                        aAttributeName == nsGkAtoms::height) &&
+                        aTargetElement->GetNameSpaceID() == kNameSpaceID_SVG &&
+                        !aTargetElement->IsAttributeMapped(aAttributeName);
+
+  return !animateAsAttr;
+}
+
 // 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());
-    }
+
+  if (IsCSSPropertyAnimation(mKey.mAttributeName, propID, mKey.mElement)) {
+    return MakeUnique<nsSMILCSSProperty>(propID, mKey.mElement.get());
   }
 
   return mKey.mElement->GetAnimatedAttr(mKey.mAttributeNamespaceID,
                                         mKey.mAttributeName);
 }
 
 uint32_t
 nsSMILCompositor::GetFirstFuncToAffectSandwich()

--- a/dom/smil/nsSMILCompositor.h
+++ b/dom/smil/nsSMILCompositor.h
@@ -67,16 +67,27 @@ 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.
+  //
+  // (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,
+                                     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
   // sandwich. Also toggles the 'mForceCompositing' flag if it finds that any
   // (used) functions have changed.
   uint32_t GetFirstFuncToAffectSandwich();