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

Source/core/css/resolver/ScopedStyleResolver.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2012 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:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 12 matching lines @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef ScopedStyleResolver_h
 #define ScopedStyleResolver_h
 
 #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"
 
 namespace WebCore {
 
 class MediaQueryEvaluator;
 class PageRuleCollector;
 class ShadowRoot;
 class StyleSheetContents;
 
 // This class selects a RenderStyle for a given element based on a collection of stylesheets.
 class ScopedStyleResolver {
     WTF_MAKE_NONCOPYABLE(ScopedStyleResolver); WTF_MAKE_FAST_ALLOCATED;
 public:
     static PassOwnPtr<ScopedStyleResolver> create(ContainerNode& scopingNode) { return adoptPtr(new ScopedStyleResolver(scopingNode)); }
 
     static ContainerNode* scopingNodeFor(Document&, const CSSStyleSheet*);
 
     const ContainerNode& scopingNode() const { return m_scopingNode; }
     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*);
     const StyleRuleKeyframes* keyframeStylesForAnimation(const StringImpl* animationName);
     void addKeyframeStyle(PassRefPtr<StyleRuleKeyframes>);
 
     void collectMatchingAuthorRules(ElementRuleCollector&, bool includeEmptyRules, bool applyAuthorStyles, CascadeScope, CascadeOrder = ignoreCascadeOrder);
     void matchPageRules(PageRuleCollector&);
     void addRulesFromSheet(CSSStyleSheet*, const MediaQueryEvaluator&, StyleResolver*);
     void collectFeaturesTo(RuleFeatureSet&);
     void resetAuthorStyle();
     void collectViewportRulesTo(StyleResolver*) const;
 
 private:
     explicit ScopedStyleResolver(ContainerNode& scopingNode) : m_scopingNode(scopingNode), m_parent(0) { }
 
     ContainerNode& m_scopingNode;
     ScopedStyleResolver* m_parent;
 
     Vector<CSSStyleSheet*> m_authorStyleSheets;
 
     typedef HashMap<const StringImpl*, RefPtr<StyleRuleKeyframes> > KeyframesRuleMap;
     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