Selectors in styles in shadowRoots should match in all of the host's shadowRoots.

Source/core/css/resolver/ScopedStyleResolver.h

Index: Source/core/css/resolver/ScopedStyleResolver.h
diff --git a/Source/core/css/resolver/ScopedStyleResolver.h b/Source/core/css/resolver/ScopedStyleResolver.h
index 2ad0d7f728c5ea954cf3424561572317f7b4cb9f..68c8e9152ec1748978c2dcb9a696a2dca59ff3e7 100644
--- a/Source/core/css/resolver/ScopedStyleResolver.h
+++ b/Source/core/css/resolver/ScopedStyleResolver.h
@@ -30,6 +30,7 @@
 #include "core/css/ElementRuleCollector.h"
 #include "core/css/RuleSet.h"
 #include "core/dom/ContainerNode.h"
+#include "core/dom/shadow/ShadowRoot.h"
 #include "wtf/HashMap.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/PassOwnPtr.h"
@@ -53,6 +54,7 @@ public:
     const TreeScope& treeScope() const { return m_scopingNode.treeScope(); }
     void setParent(ScopedStyleResolver* newParent) { m_parent = newParent; }
     ScopedStyleResolver* parent() { return m_parent; }
+    bool crossingScopeBoundary(const Element*) const;
 
 public:
     bool checkRegionStyle(Element*);
@@ -78,6 +80,30 @@ private:
     KeyframesRuleMap m_keyframesRuleMap;
 };
 
+inline bool ScopedStyleResolver::crossingScopeBoundary(const Element* element) const
+{
+    // Since this resolver is for style scoped, rules declared in the style have higher priority
+    // than rules declared in styles whose scoping node is the ancestor. So need to ++cascadeScope.
+    if (!m_scopingNode.isShadowRoot())
+        return true;
+
+    // If parent is a given shadow host, this resolver is for shadow trees hosted by the host.
+    // Since styles in shadow trees hosted by the same shadow host will apply all elements in
+    // the shadow trees, we should treat rules in the styles as the same cascade.
+    // Should not ++cascadeScope.
+    Element* host = toShadowRoot(&m_scopingNode)->host();
+    if (element == host)
+        return false;
+
+    // If parent is document scope or style scoped, ++cascadeScope because of CSS cascade spec.
+    if (!m_parent || !m_parent->scopingNode().isShadowRoot())
+        return true;
+
+    // If parent is hosted by the same shadow host, keep the same cascadeScope.
+    // Otherwise, ++cascadeScope.
+    return toShadowRoot(&m_parent->scopingNode())->host() != host;
+}
+
 } // namespace WebCore
 
 #endif // ScopedStyleResolver_h