Adding @keyframes rules only affects TreeScope plus host.

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

Index: third_party/WebKit/Source/core/animation/css/CSSAnimations.cpp
diff --git a/third_party/WebKit/Source/core/animation/css/CSSAnimations.cpp b/third_party/WebKit/Source/core/animation/css/CSSAnimations.cpp
index 9de013e0504219538784dd11a15444359931adb5..5eff4e8c0a6e852dced2c52ebf1a5f8a4fe11d43 100644
--- a/third_party/WebKit/Source/core/animation/css/CSSAnimations.cpp
+++ b/third_party/WebKit/Source/core/animation/css/CSSAnimations.cpp
@@ -306,10 +306,8 @@ void CSSAnimations::calculateAnimationUpdate(CSSAnimationUpdate& update, const E
             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;
@@ -896,6 +894,19 @@ bool CSSAnimations::isAnimatableProperty(CSSPropertyID property)
     }
 }
 
+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);