Implement drop-down menu with HTML+CSS

nbsp;html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">    
    
    
<meta>    
<title>下拉框</title>    
<style>    
*{ margin:0px; padding:0px;}    
#nav{ width:600px; height:40px; margin:0 auto;}    
#nav ul{ list-style:none;}    
#nav ul li{ float:left; line-height:40px; text-align:center; position:relative;}    
#nav ul li a{ text-decoration:none; color:#000; display:block;padding:0px 10px;}    
#nav ul li a:hover{ color:#FFF; background:#333}    
#nav ul li ul{ position:absolute; display:none;}    
#nav ul li ul li{ float:none; line-height:30px; text-align:left;}    
#nav ul li ul li a{ width:100%;}    
#nav ul li ul li a:hover{ background-color:#06f;}    
#nav ul li:hover ul{ display:block}    
</style>    
    
<p>    
    </p>