[import] The first <link rel=import> should win in the cascading order computation.

Source/core/dom/DocumentStyleSheetCollector.h

 /*
  * Copyright (C) 2014 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Neither the name of Google Inc. nor the names of its
  * contributors may be used to endorse or promote products derived from
  * this software without specific prior written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (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 DocumentStyleSheetCollector_h
 #define DocumentStyleSheetCollector_h
 
+#include "wtf/HashSet.h"
 #include "wtf/RefPtr.h"
 #include "wtf/Vector.h"
 
 namespace WebCore {
 
 class CSSStyleSheet;
+class Document;
 class StyleSheet;
 class StyleSheetCollection;
 
 class DocumentStyleSheetCollector {
 public:
     friend class ImportedDocumentStyleSheetCollector;
 
-    DocumentStyleSheetCollector(Vector<RefPtr<StyleSheet> >& sheetsForList, Vector<RefPtr<CSSStyleSheet> >& activeList);
+    DocumentStyleSheetCollector(Vector<RefPtr<StyleSheet> >& sheetsForList, Vector<RefPtr<CSSStyleSheet> >& activeList, HashSet<Document*>&);
     ~DocumentStyleSheetCollector();
 
     void appendActiveStyleSheets(const Vector<RefPtr<CSSStyleSheet> >&);
     void appendActiveStyleSheet(CSSStyleSheet*);
     void appendSheetForList(StyleSheet*);
 
+    bool hasVisited(Document* document) const { return m_visitedDocuments.contains(document); }
+    void willVisit(Document* document) { m_visitedDocuments.add(document); }
+
 private:
     Vector<RefPtr<StyleSheet> >& m_styleSheetsForStyleSheetList;
     Vector<RefPtr<CSSStyleSheet> >& m_activeAuthorStyleSheets;
+    HashSet<Document*>& m_visitedDocuments;
 };
 
 class ActiveDocumentStyleSheetCollector FINAL : public DocumentStyleSheetCollector {
 public:
     ActiveDocumentStyleSheetCollector(StyleSheetCollection&);
+private:
+    HashSet<Document*> m_visitedDocuments;
 };
 
 class ImportedDocumentStyleSheetCollector FINAL : public DocumentStyleSheetCollector {
 public:
     ImportedDocumentStyleSheetCollector(DocumentStyleSheetCollector&, Vector<RefPtr<StyleSheet> >&);
 };
 
 } // namespace WebCore
 
 #endif // DocumentStyleSheetCollector_h