--- a/dom/svg/nsSVGElement.cpp
+++ b/dom/svg/nsSVGElement.cpp
@@ -123,24 +123,34 @@ nsSVGElement::GetStyle(nsIDOMCSSStyleDec
 }
 
 //----------------------------------------------------------------------
 // nsSVGElement methods
 
 void
 nsSVGElement::DidAnimateClass()
 {
+  // For Servo, snapshot the element before we change it.
+  nsIPresShell* shell = OwnerDoc()->GetShell();
+  if (shell) {
+    nsPresContext* presContext = shell->GetPresContext();
+    if (presContext && presContext->RestyleManager()->IsServo()) {
+      presContext->RestyleManager()
+                  ->AsServo()
+                  ->ClassAttributeWillBeChangedBySMIL(this);
+    }
+  }
+
   nsAutoString src;
   mClassAttribute.GetAnimValue(src, this);
   if (!mClassAnimAttr) {
     mClassAnimAttr = new nsAttrValue();
   }
   mClassAnimAttr->ParseAtomArray(src);
 
-  nsIPresShell* shell = OwnerDoc()->GetShell();
   if (shell) {
     shell->RestyleForAnimation(this, eRestyle_Self);
   }
 }
 
 nsresult
 nsSVGElement::Init()
 {

--- a/layout/base/ServoRestyleManager.cpp
+++ b/layout/base/ServoRestyleManager.cpp
@@ -831,16 +831,30 @@ AttributeInfluencesOtherPseudoClassState
 }
 
 void
 ServoRestyleManager::AttributeWillChange(Element* aElement,
                                          int32_t aNameSpaceID,
                                          nsIAtom* aAttribute, int32_t aModType,
                                          const nsAttrValue* aNewValue)
 {
+  SnapshotForAttributeChange(aElement, aNameSpaceID, aAttribute);
+}
+
+void
+ServoRestyleManager::ClassAttributeWillBeChangedBySMIL(Element* aElement)
+{
+  SnapshotForAttributeChange(aElement, kNameSpaceID_None, nsGkAtoms::_class);
+}
+
+void
+ServoRestyleManager::SnapshotForAttributeChange(Element* aElement,
+                                                int32_t aNameSpaceID,
+                                                nsIAtom* aAttribute)
+{
   MOZ_ASSERT(!mInStyleRefresh);
 
   if (!aElement->HasServoData()) {
     return;
   }
 
   bool influencesOtherPseudoClassState =
     AttributeInfluencesOtherPseudoClassState(aElement, aAttribute);

--- a/layout/base/ServoRestyleManager.h
+++ b/layout/base/ServoRestyleManager.h
@@ -64,16 +64,17 @@ public:
   void RestyleForAppend(nsIContent* aContainer,
                         nsIContent* aFirstNewContent);
   void ContentStateChanged(nsIContent* aContent, EventStates aStateMask);
   void AttributeWillChange(dom::Element* aElement,
                            int32_t aNameSpaceID,
                            nsIAtom* aAttribute,
                            int32_t aModType,
                            const nsAttrValue* aNewValue);
+  void ClassAttributeWillBeChangedBySMIL(dom::Element* aElement);
 
   void AttributeChanged(dom::Element* aElement, int32_t aNameSpaceID,
                         nsIAtom* aAttribute, int32_t aModType,
                         const nsAttrValue* aOldValue);
 
   nsresult ReparentStyleContext(nsIFrame* aFrame);
 
   /**
@@ -142,16 +143,19 @@ private:
                "ServoRestyleManager should only be used with a Servo-flavored "
                "style backend");
     return PresContext()->StyleSet()->AsServo();
   }
 
   const SnapshotTable& Snapshots() const { return mSnapshots; }
   void ClearSnapshots();
   ServoElementSnapshot& SnapshotFor(mozilla::dom::Element* aElement);
+  void SnapshotForAttributeChange(mozilla::dom::Element* aElement,
+                                  int32_t aNameSpaceID,
+                                  nsIAtom* aAttribute);
 
   void DoProcessPendingRestyles(TraversalRestyleBehavior aRestyleBehavior);
 
   // We use a separate data structure from nsStyleChangeList because we need a
   // frame to create nsStyleChangeList entries, and the primary frame may not be
   // attached yet.
   struct ReentrantChange {
     nsCOMPtr<nsIContent> mContent;