Implement 'mouseenter' and 'mouseleave' from DOM Level 3 Events.

Source/core/dom/Element.idl

 /*
  * Copyright (C) 2006, 2007, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * 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.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 127 matching lines @@
     [NotEnumerable, PerWorldBindings] attribute EventListener ondrop;
     [NotEnumerable, PerWorldBindings] attribute EventListener onerror;
     [NotEnumerable, PerWorldBindings] attribute EventListener onfocus;
     [NotEnumerable, PerWorldBindings] attribute EventListener oninput;
     [NotEnumerable, PerWorldBindings] attribute EventListener oninvalid;
     [NotEnumerable, PerWorldBindings] attribute EventListener onkeydown;
     [NotEnumerable, PerWorldBindings] attribute EventListener onkeypress;
     [NotEnumerable, PerWorldBindings] attribute EventListener onkeyup;
     [NotEnumerable, PerWorldBindings] attribute EventListener onload;
     [NotEnumerable, PerWorldBindings] attribute EventListener onmousedown;
+    [NotEnumerable, PerWorldBindings] attribute EventListener onmouseenter;
+    [NotEnumerable, PerWorldBindings] attribute EventListener onmouseleave;
     [NotEnumerable, PerWorldBindings] attribute EventListener onmousemove;
     [NotEnumerable, PerWorldBindings] attribute EventListener onmouseout;
     [NotEnumerable, PerWorldBindings] attribute EventListener onmouseover;
     [NotEnumerable, PerWorldBindings] attribute EventListener onmouseup;
     [NotEnumerable, PerWorldBindings] attribute EventListener onmousewheel;
     [NotEnumerable, PerWorldBindings] attribute EventListener onscroll;
     [NotEnumerable, PerWorldBindings] attribute EventListener onselect;
     [NotEnumerable, PerWorldBindings] attribute EventListener onsubmit;
 
     // attribute [NotEnumerable] EventListener oncanplay;
@@ skipping 33 matching lines @@
     [NotEnumerable, EnabledAtRuntime=touch, PerWorldBindings] attribute EventListener ontouchmove;
     [NotEnumerable, EnabledAtRuntime=touch, PerWorldBindings] attribute EventListener ontouchend;
     [NotEnumerable, EnabledAtRuntime=touch, PerWorldBindings] attribute EventListener ontouchcancel;
     [NotEnumerable, PerWorldBindings] attribute EventListener onwebkitfullscreenchange;
     [NotEnumerable, PerWorldBindings] attribute EventListener onwebkitfullscreenerror;
 };
 
 Element implements ParentNode;
 Element implements ChildNode;