Make getProperty more strict when handling the priority parameter

Source/core/css/PropertySetCSSStyleDeclaration.cpp

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Research In Motion Limited. 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.
  *
@@ skipping 197 matching lines @@
     return propertySet()->isPropertyImplicit(propertyID);
 }
 
 void AbstractPropertySetCSSStyleDeclaration::setProperty(const String& propertyName, const String& value, const String& priority, ExceptionState& exceptionState)
 {
     StyleAttributeMutationScope mutationScope(this);
     CSSPropertyID propertyID = cssPropertyID(propertyName);
     if (!propertyID)
         return;
 
-    bool important = priority.find("important", 0, false) != kNotFound;
+    bool important = equalIgnoringCase(priority, "important");
+    if (!important && !priority.isEmpty())
+        return;
 
     willMutate();
 
     bool changed = propertySet()->setProperty(propertyID, value, important, contextStyleSheet());
 
     didMutate(changed ? PropertyChanged : NoChanges);
 
     if (changed) {
         // CSS DOM requires raising SyntaxError of parsing failed, but this is too dangerous for compatibility,
         // see <http://bugs.webkit.org/show_bug.cgi?id=7296>.
@@ skipping 159 matching lines @@
 {
     m_parentElement->ref();
 }
 
 void InlineCSSStyleDeclaration::deref()
 {
     m_parentElement->deref();
 }
 
 } // namespace WebCore