--- a/dom/animation/KeyframeEffectReadOnly.cpp
+++ b/dom/animation/KeyframeEffectReadOnly.cpp
@@ -206,17 +206,18 @@ KeyframeEffectReadOnly::SetKeyframes(
 
   mKeyframes = Move(aKeyframes);
   KeyframeUtils::DistributeKeyframes(mKeyframes);
 
   if (mAnimation && mAnimation->IsRelevant()) {
     nsNodeUtils::AnimationChanged(mAnimation);
   }
 
-  // We need to call UpdateProperties() if the StyleType is not nullptr.
+  // We need to call UpdateProperties() unless the target element doesn't have
+  // style (e.g. the target element is not associated with any document).
   if (aStyle) {
     UpdateProperties(aStyle);
     MaybeUpdateFrameForCompositor();
   }
 }
 
 const AnimationProperty*
 KeyframeEffectReadOnly::GetEffectiveAnimationOfProperty(