为什么window的父窗口为空?
我写了如下的javascript   
  <!DOCTYPE   html   PUBLIC    "-//W3C//DTD   HTML   4.01   Transitional//EN "    "http://www.w3.org/TR/html4/loose.dtd ">  
  <html>  
 	 <head>  
 		 <meta   http-equiv= "Content-Type "   content= "text/html;   charset=UTF-8 ">  
 		 <title> Insert   title   here </title>  
 	 </head>  
 	 <body>  
 		 <a   href= "http://www.163.com "   target= "_parent "> www.163.com </a>  
 		 <button   onclick= "display() "> display   parent </button>  
 		 <script   type= "text/javascript ">  <!-- 
 			function   display(){ 
 				var   parent   =   this.parent; 
 				var   i   =   1; 
 				while(parent      !=   null){ 
 					alert(i); 
 					alert(parent); 
 					i++; 
 					if(   i   ==   10){ 
 						break; 
 					} 
 					parent   =   parent.parent; 
 				} 
 			}   
                         -->  </script>  
 	 </body>  
  </html>      
 可是,为什么window.parent却永远不为null呢?奇怪!
------解决方案--------------------顶层窗口引用的是本身