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 566 matching lines @@
     case CSSPropertyTransitionDelay:
     case CSSPropertyTransitionDuration:
     case CSSPropertyTransitionProperty:
     case CSSPropertyTransitionTimingFunction:
         return false;
     default:
         return true;
     }
 }
 
+bool CSSAnimations::isAffectedByKeyframesFromScope(const Element& element, const TreeScope& treeScope)
+{
+    // Animated elements are affected by @keyframes rules from the same scope
+    // and from their shadow sub-trees if they are shadow hosts.
+    if (element.treeScope() == treeScope)
+        return true;
+    if (!isShadowHost(element))
+        return false;
+    if (treeScope.rootNode() == treeScope.document())
+        return false;
+    return toShadowRoot(treeScope.rootNode()).host() == element;
+}
+
 DEFINE_TRACE(CSSAnimations)
 {
     visitor->trace(m_transitions);
     visitor->trace(m_pendingUpdate);
     visitor->trace(m_runningAnimations);
 }
 
 } // namespace blink