--- a/dom/animation/KeyframeEffectReadOnly.cpp
+++ b/dom/animation/KeyframeEffectReadOnly.cpp
@@ -272,16 +272,18 @@ SpecifiedKeyframeArraysAreEqual(const ns
 void
 KeyframeEffectReadOnly::UpdateProperties(nsStyleContext* aStyleContext)
 {
   MOZ_ASSERT(aStyleContext);
 
   // Skip updating properties when we are composing style.
   // FIXME: Bug 1324966. Drop this check once we have a function to get
   // nsStyleContext without resolving animating style.
+  MOZ_DIAGNOSTIC_ASSERT(!mIsComposingStyle,
+                        "Should not be called while processing ComposeStyle()");
   if (mIsComposingStyle) {
     return;
   }
 
   nsTArray<AnimationProperty> properties = BuildProperties(aStyleContext);
 
   if (mProperties == properties) {
     return;
@@ -460,16 +462,18 @@ KeyframeEffectReadOnly::EnsureBaseStyles
   }
 }
 
 void
 KeyframeEffectReadOnly::ComposeStyle(
   RefPtr<AnimValuesStyleRule>& aStyleRule,
   const nsCSSPropertyIDSet& aPropertiesToSkip)
 {
+  MOZ_DIAGNOSTIC_ASSERT(!mIsComposingStyle,
+                        "Should not be called recursively");
   if (mIsComposingStyle) {
     return;
   }
 
   AutoRestore<bool> isComposingStyle(mIsComposingStyle);
   mIsComposingStyle = true;
 
   ComputedTiming computedTiming = GetComputedTiming();