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

Source/core/css/ElementRuleCollector.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.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 36 matching lines @@
 const CascadeScope ignoreCascadeScope = 0;
 const CascadeOrder ignoreCascadeOrder = 0;
 
 class MatchedRule {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     explicit MatchedRule(const RuleData* ruleData, unsigned specificity, CascadeScope cascadeScope, CascadeOrder cascadeOrder, unsigned styleSheetIndex, const CSSStyleSheet* parentStyleSheet)
         : m_ruleData(ruleData)
         , m_specificity(specificity)
         , m_cascadeScope(cascadeScope)
-        , m_styleSheetIndex(styleSheetIndex)
         , m_parentStyleSheet(parentStyleSheet)
     {
         ASSERT(m_ruleData);
         static const unsigned BitsForPositionInRuleData = 18;
-        m_position = (cascadeOrder << BitsForPositionInRuleData) + m_ruleData->position();
+        static const unsigned BitsForStyleSheetIndexInRuleData = 32;
+        m_position = ((uint64_t)cascadeOrder << (BitsForStyleSheetIndexInRuleData + BitsForPositionInRuleData)) + ((uint64_t)styleSheetIndex << BitsForPositionInRuleData) + m_ruleData->position();
     }
 
     const RuleData* ruleData() const { return m_ruleData; }
     uint32_t cascadeScope() const { return m_cascadeScope; }
     uint32_t position() const { return m_position; }
     unsigned specificity() const { return ruleData()->specificity() + m_specificity; }
-    uint32_t styleSheetIndex() const { return m_styleSheetIndex; }
     const CSSStyleSheet* parentStyleSheet() const { return m_parentStyleSheet; }
 
 private:
     const RuleData* m_ruleData;
     unsigned m_specificity;
     CascadeScope m_cascadeScope;
-    uint32_t m_position;
-    uint32_t m_styleSheetIndex;
     const CSSStyleSheet* m_parentStyleSheet;
+    uint64_t m_position;
 };
 
 class StyleRuleList : public RefCounted<StyleRuleList> {
 public:
     static PassRefPtr<StyleRuleList> create() { return adoptRef(new StyleRuleList()); }
     Vector<StyleRule*> m_list;
 };
 
 // ElementRuleCollector is designed to be used as a stack object.
 // Create one, ask what rules the ElementResolveContext matches
@@ skipping 60 matching lines @@
 
     // Output.
     RefPtr<StaticCSSRuleList> m_cssRuleList;
     RefPtr<StyleRuleList> m_styleRuleList;
     MatchResult m_result;
 };
 
 } // namespace WebCore
 
 #endif // ElementRuleCollector_h