Adding @keyframes rules only affects TreeScope plus host.

third_party/WebKit/Source/core/animation/css/CSSAnimations.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 288 matching lines @@
             }
 
             const bool isPaused = CSSTimingData::getRepeated(animationData->playStateList(), i) == AnimPlayStatePaused;
 
             Timing timing = animationData->convertToTiming(i);
             Timing specifiedTiming = timing;
             RefPtr<TimingFunction> keyframeTimingFunction = timing.timingFunction;
             timing.timingFunction = Timing::defaults().timingFunction;
 
             StyleRuleKeyframes* keyframesRule = resolver->findKeyframesRule(elementForScoping, name);
-            if (!keyframesRule) {
-                element.document().styleEngine().setHasUnresolvedKeyframesRule();
+            if (!keyframesRule)
                 continue; // Cancel the animation if there's no style rule for it.
-            }
 
             const RunningAnimation* existingAnimation = nullptr;
             size_t existingAnimationIndex = 0;
 
             if (cssAnimations) {
                 for (size_t i = 0; i < cssAnimations->m_runningAnimations.size(); i++) {
                     const RunningAnimation& runningAnimation = *cssAnimations->m_runningAnimations[i];
                     if (runningAnimation.name == name && runningAnimation.nameIndex == nameIndex) {
                         existingAnimation = &runningAnimation;
                         existingAnimationIndex = i;
@@ skipping 574 matching lines @@
 }
 
 DEFINE_TRACE(CSSAnimations)
 {
     visitor->trace(m_transitions);
     visitor->trace(m_pendingUpdate);
     visitor->trace(m_runningAnimations);
 }
 
 } // namespace blink