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

Source/core/dom/StyleEngine.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 213 matching lines @@
 
 void StyleEngine::invalidateInjectedStyleSheetCache()
 {
     m_injectedStyleSheetCacheValid = false;
     markDocumentDirty();
     // FIXME: updateInjectedStyleSheetCache is called inside StyleSheetCollection::updateActiveStyleSheets
     // and batch updates lots of sheets so we can't call addedStyleSheet() or removedStyleSheet().
     document().styleResolverChanged();
 }
 
+void StyleEngine::compatibilityModeChanged()
+{
+    if (!m_injectedAuthorStyleSheets.isEmpty())

[esprehn, 2014/08/19 11:30:59]

Is this correct? If the cache is empty why don't we need to recompute it and set
the dirty bit in there?

[rune, 2014/08/19 13:31:16]

The cache is the set of injected stylesheet which apply to the url of the
document for this style engine. That cache needs to be updated when the list of
injected stylesheets change since there might be stylesheets in the cache which
are removed from the global list, or newly added injected stylesheets which may
match the url of this document.

For a compat mode change, however, the list of injected stylesheets doesn't
change, so the list of matching stylesheets (the cache) should not change, but
their contents need to be reparsed. If the cache is empty, there are no
stylesheets to reparse.

A question is if injected stylesheets really should or need follow the
compatMode of the document into which they are inserted.

+        invalidateInjectedStyleSheetCache();
+}
+
 void StyleEngine::addAuthorSheet(PassRefPtrWillBeRawPtr<StyleSheetContents> authorSheet)
 {
     m_authorStyleSheets.append(CSSStyleSheet::create(authorSheet, m_document));
     document().addedStyleSheet(m_authorStyleSheets.last().get());
     markDocumentDirty();
 }
 
 void StyleEngine::addPendingSheet()
 {
     m_pendingStylesheets++;
@@ skipping 461 matching lines @@
     visitor->trace(m_resolver);
     visitor->trace(m_fontSelector);
     visitor->trace(m_textToSheetCache);
     visitor->trace(m_sheetToTextCache);
     visitor->trace(m_xslStyleSheet);
 #endif
     CSSFontSelectorClient::trace(visitor);
 }
 
 }