diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index c24a88d958b0..cd4bd09e0875 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -1835,11 +1835,8 @@ DeclarationBlock* Element::GetSMILOverrideStyleDeclaration() {
   return slots ? slots->mSMILOverrideStyleDeclaration.get() : nullptr;
 }
 
-nsresult Element::SetSMILOverrideStyleDeclaration(
-    DeclarationBlock* aDeclaration) {
-  Element::nsExtendedDOMSlots* slots = ExtendedDOMSlots();
-
-  slots->mSMILOverrideStyleDeclaration = aDeclaration;
+void Element::SetSMILOverrideStyleDeclaration(DeclarationBlock& aDeclaration) {
+  ExtendedDOMSlots()->mSMILOverrideStyleDeclaration = &aDeclaration;
 
   // Only need to request a restyle if we're in a document.  (We might not
   // be in a document, if we're clearing animation effects on a target node
@@ -1849,8 +1846,6 @@ nsresult Element::SetSMILOverrideStyleDeclaration(
       presShell->RestyleForAnimation(this, StyleRestyleHint_RESTYLE_SMIL);
     }
   }
-
-  return NS_OK;
 }
 
 bool Element::IsLabelable() const { return false; }
diff --git a/dom/base/Element.h b/dom/base/Element.h
index 598f8a718953..e07e06a5b9e5 100644
--- a/dom/base/Element.h
+++ b/dom/base/Element.h
@@ -336,7 +336,7 @@ class Element : public FragmentOrElement {
    * notify the document's pres context, so that the style changes will be
    * noticed.
    */
-  nsresult SetSMILOverrideStyleDeclaration(DeclarationBlock* aDeclaration);
+  void SetSMILOverrideStyleDeclaration(DeclarationBlock&);
 
   /**
    * Returns a new SMILAttr that allows the caller to animate the given
diff --git a/layout/style/nsDOMCSSAttrDeclaration.cpp b/layout/style/nsDOMCSSAttrDeclaration.cpp
index b131e9d87aa4..9b5d52728d3b 100644
--- a/layout/style/nsDOMCSSAttrDeclaration.cpp
+++ b/layout/style/nsDOMCSSAttrDeclaration.cpp
@@ -79,9 +79,11 @@ nsresult nsDOMCSSAttributeDeclaration::SetCSSDeclaration(
   MOZ_ASSERT_IF(aClosureData, !aClosureData->mClosure);
 
   aDecl->SetDirty();
-  return mIsSMILOverride
-             ? mElement->SetSMILOverrideStyleDeclaration(aDecl)
-             : mElement->SetInlineStyleDeclaration(*aDecl, *aClosureData);
+  if (mIsSMILOverride) {
+    mElement->SetSMILOverrideStyleDeclaration(*aDecl);
+    return NS_OK;
+  }
+  return mElement->SetInlineStyleDeclaration(*aDecl, *aClosureData);
 }
 
 Document* nsDOMCSSAttributeDeclaration::DocToUpdate() {