diff --git a/content/smil/nsSMILAnimationFunction.cpp b/content/smil/nsSMILAnimationFunction.cpp
index 13cb38ecf75..26fc643516c 100644
--- a/content/smil/nsSMILAnimationFunction.cpp
+++ b/content/smil/nsSMILAnimationFunction.cpp
@@ -360,6 +360,16 @@ nsSMILAnimationFunction::HasChanged() const
   return mHasChanged;
 }
 
+PRBool
+nsSMILAnimationFunction::UpdateCachedTarget(const nsSMILTargetIdentifier& aNewTarget)
+{
+  if (!mLastTarget.Equals(aNewTarget)) {
+    mLastTarget = aNewTarget;
+    return PR_TRUE;
+  }
+  return PR_FALSE;
+}
+
 //----------------------------------------------------------------------
 // Implementation helpers
 
diff --git a/content/smil/nsSMILAnimationFunction.h b/content/smil/nsSMILAnimationFunction.h
index 877b002b9bb..318dc3fd7fb 100644
--- a/content/smil/nsSMILAnimationFunction.h
+++ b/content/smil/nsSMILAnimationFunction.h
@@ -42,6 +42,7 @@
 #include "nsISMILAttr.h"
 #include "nsGkAtoms.h"
 #include "nsString.h"
+#include "nsSMILTargetIdentifier.h"
 #include "nsSMILTimeValue.h"
 #include "nsSMILKeySpline.h"
 #include "nsSMILValue.h"
@@ -207,14 +208,28 @@ public:
    * time it was composited. This allows rendering to be performed only when
    * necessary, particularly when no animations are active.
    *
-   * Note that the caller is responsible for determining if the animation target
-   * has changed.
+   * Note that the caller is responsible for determining if the animation
+   * target has changed (with help from my UpdateCachedTarget() method).
    *
    * @return  PR_TRUE if the animation parameters have changed, PR_FALSE
    *          otherwise.
    */
   PRBool HasChanged() const;
 
+  /**
+   * Updates the cached record of our animation target, and returns a boolean
+   * that indicates whether the target has changed since the last call to this
+   * function. (This lets nsSMILCompositor check whether its animation
+   * functions have changed value or target since the last sample.  If none of
+   * them have, then the compositor doesn't need to do anything.)
+   *
+   * @param aNewTarget A nsSMILTargetIdentifier representing the animation
+   *                   target of this function for this sample.
+   * @return  PR_TRUE if |aNewTarget| is different from the old cached value;
+   *          otherwise, PR_FALSE.
+   */
+  PRBool UpdateCachedTarget(const nsSMILTargetIdentifier& aNewTarget);
+
   // Comparator utility class, used for sorting nsSMILAnimationFunctions
   class Comparator {
     public:
@@ -378,6 +393,11 @@ protected:
   // @see
   // http://www.w3.org/TR/2001/REC-smil-animation-20010904/#FromToByAndAdditive
   nsSMILValue                   mFrozenValue;
+
+  // Allows us to check whether an animation function has changed target from
+  // sample to sample (because if neither target nor animated value have
+  // changed, we don't have to do anything).
+  nsSMILWeakTargetIdentifier    mLastTarget;
 };
 
 #endif // NS_SMILANIMATIONFUNCTION_H_
diff --git a/content/smil/nsSMILCompositor.cpp b/content/smil/nsSMILCompositor.cpp
index 98c3ec81558..3d094210570 100644
--- a/content/smil/nsSMILCompositor.cpp
+++ b/content/smil/nsSMILCompositor.cpp
@@ -114,24 +114,15 @@ nsSMILCompositor::ComposeAttribute()
 
   // THIRD: Step backwards through animation functions to find out
   // which ones we actually care about.
-  //  PRBool changed = PR_FALSE; // XXXdholbert removing until we have
-                                 //  HasChangedTarget
+  PRBool changed = PR_FALSE;
   PRUint32 length = mAnimationFunctions.Length();
   PRUint32 i;
   for (i = length; i > 0; --i) {
     nsSMILAnimationFunction* curAnimFunc = mAnimationFunctions[i-1];
-    // XXXdholbert we need to add another function
-    // nsSMILAnimationFunction::HasChangedTarget(elem, smilAttr, isCSS) that
-    // we call here (in addition to HasChanged(), because even if function
-    // value hasn't changed, its target might have.
-    // For this to work, the nsSMILAnimationFunction needs to cache its last
-    // elem/smilAttr/isCSS values, and then check them against the new values
-    // here.
-    /*
-    if (!changed && curAnimFunc->HasChanged()) {
+    if (curAnimFunc->UpdateCachedTarget(mKey) ||
+        (!changed && curAnimFunc->HasChanged())) {
       changed = PR_TRUE;
     }
-    */
 
     if (curAnimFunc->WillReplace()) {
       --i;
@@ -141,7 +132,7 @@ nsSMILCompositor::ComposeAttribute()
   // NOTE: 'i' is now the index of the first animation function that we need
   // to use in compositing.
 
-  // if (!changed) // XXXdholbert removing until we have HasChangedTarget
+  // if (!changed) // XXXdholbert Still need to enable this optimization
   //  return;
 
   // FOURTH: Compose animation functions (starting with base value)