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 f5f3fc0f9e1ecde24f85d2337b2c1f2f81ea66fa..fcdf465b36f5d3e98123e0e8e2713f7c75b1d64c 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;