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

Source/core/css/ElementRuleCollector.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 360 matching lines @@
 static inline bool compareRules(const MatchedRule& matchedRule1, const MatchedRule& matchedRule2)
 {
     if (matchedRule1.cascadeScope() != matchedRule2.cascadeScope())
         return matchedRule1.cascadeScope() > matchedRule2.cascadeScope();
 
     unsigned specificity1 = matchedRule1.specificity();
     unsigned specificity2 = matchedRule2.specificity();
     if (specificity1 != specificity2)
         return specificity1 < specificity2;
 
-    if (matchedRule1.styleSheetIndex() != matchedRule2.styleSheetIndex())
-        return matchedRule1.styleSheetIndex() < matchedRule2.styleSheetIndex();
-
     return matchedRule1.position() < matchedRule2.position();
 }
 
 void ElementRuleCollector::sortMatchedRules()
 {
     ASSERT(m_matchedRules);
     std::sort(m_matchedRules->begin(), m_matchedRules->end(), compareRules);
 }
 
 bool ElementRuleCollector::hasAnyMatchingRules(RuleSet* ruleSet)
 {
     clearMatchedRules();
 
     m_mode = SelectorChecker::SharingRules;
     // To check whether a given RuleSet has any rule matching a given element,
     // should not see the element's treescope. Because RuleSet has no
     // information about "scope".
     int firstRuleIndex = -1, lastRuleIndex = -1;
     RuleRange ruleRange(firstRuleIndex, lastRuleIndex);
     // FIXME: Verify whether it's ok to ignore CascadeScope here.
     collectMatchingRules(MatchRequest(ruleSet), ruleRange, SelectorChecker::StaysWithinTreeScope);
 
     return m_matchedRules && !m_matchedRules->isEmpty();
 }
 
 } // namespace WebCore