No need to re-parse empty set of injected stylesheets.

Source/core/dom/Document.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 691 matching lines @@
 
 void Document::setCompatibilityMode(CompatibilityMode mode)
 {
     if (m_compatibilityModeLocked || mode == m_compatibilityMode)
         return;
     bool wasInQuirksMode = inQuirksMode();
     m_compatibilityMode = mode;
     selectorQueryCache().invalidate();
     if (inQuirksMode() != wasInQuirksMode) {
         // All injected stylesheets have to reparse using the different mode.
-        m_styleEngine->invalidateInjectedStyleSheetCache();
+        m_styleEngine->compatibilityModeChanged();
     }
 }
 
 String Document::compatMode() const
 {
     return inQuirksMode() ? "BackCompat" : "CSS1Compat";
 }
 
 void Document::setDoctype(PassRefPtrWillBeRawPtr<DocumentType> docType)
 {
@@ skipping 5160 matching lines @@
 using namespace blink;
 void showLiveDocumentInstances()
 {
     WeakDocumentSet& set = liveDocumentSet();
     fprintf(stderr, "There are %u documents currently alive:\n", set.size());
     for (WeakDocumentSet::const_iterator it = set.begin(); it != set.end(); ++it) {
         fprintf(stderr, "- Document %p URL: %s\n", *it, (*it)->url().string().utf8().data());
     }
 }
 #endif