为什么列表元素的CSS样式的值是空的?
我是想输入 <ul>  <ol> 的display,margin属性.可是为什么是空的呢?   
 代码如下:     
  <!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=GB18030 ">  
 		 <title> Insert   title   here </title>  
 		 <style   type= "text/css ">  
 		ol   {   margin-left:0px;   list-style-type:lower-alpha;   list-style-position:inside;   }    
 		   ol   li{   margin-left:0px;   border:1px   solid   black;list-style-type:upper-alpha;   } 
 		   a{display:list-item;list-style-type:   decimal;      margin-left:80px;   } 
 		   a:link{color:blue;} 
 		   a:hover{color:red;} 
 		   a:active{color:yellow;} 
 		   a:visited{color:green;} 
 		 </style>  
 	 </head>  
 	 <body>  
 		 <h2   id= "h2 ">  
 			省份 
 		 </h2>  
 		 <ol   id= "a "   >  
 			 <li   id= "a1 ">  
 				湖北 
 			 </li>  
 			 <li>  
 				湖南 
 			 </li>  
 			 <li>  
 				浙江 
 			 </li>  
 		 </ol>  
 		 <h2>  
 			城市 
 		 </h2>  
 		 <ul   id= "b ">  
 			 <li   id= "b1 ">  
 				武汉 
 			 </li>  
 			 <li>  
 				长沙 
 			 </li>  
 			 <li>  
 				杭州 
 			 </li>  
 		 </ul>   	     	   	 
 		 <script   type= "text/javascript ">        	 
 		   document.writeln( "h2   display: "+document.getElementById( "h2 ").style.display+ " <br>  "); 
 		   document.writeln( "h2   margin: "+document.getElementById( "h2 ").style.margin+ " <br>  "); 
 		      document.writeln( "a   display: "+document.getElementById( "a ").style.display+ " <br>  "); 
 		   document.writeln( "a   margin: "+document.getElementById( "a ").style.margin+ " <br>  "); 
 		   document.writeln( "a   listStyleType: "+document.getElementById( "a ").style.listStyleType+ " <br>  ");  		       
 		      document.writeln( "a1   display: "+document.getElementById( "a1 ").style.display+ " <br>  "); 
 		   document.writeln( "a1   margin: "+document.getElementById( "a1 ").style.marginLeft   + " <br>  "); 
 		//   alert(document.getElementById( "a1 ").style.marginLeft); 
 		      document.writeln( "b   display: "+document.getElementById( "b ").style.display+ " <br>  "); 
 		   document.writeln( "b   margin: "+document.getElementById( "b ").style.margin+ " <br>  ");  		    
 		      document.writeln( "b1   display: "+document.getElementById( "b1 ").style.display+ " <br>  "); 
 		   document.writeln( "b1   margin: "+document.getElementById( "b1 ").style.margin+ " <br>  ");  		  		    
 		 </script>  
 	 </body>  
  <