<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>className</title>
<style type="text/css">
    li{ list-style:none;}
    .box{ width:100px; height:100px; line-height:100px; background:#CC3300; margin:4px; color:#fff; text-align:center;}
    .green{ background:#009900;}
    .selected{ background:#9900FF;}
</style>

<script type="text/javascript">
    window.onload=function(){
        var lis=document.getElementsByTagName('li');
        var i=0;
        
        for(i=0; i<lis.length; i++){
            lis[i].onmouseover=function(){
                this.className+=' selected';
            };
            
            lis[i].onmouseout=function(){
                var oldClassName=this.className;
                var oldClassNameArray=oldClassName.split(' ');
                oldClassNameArray.pop();
                var nowClassName=oldClassNameArray.join(' ');
                this.className=nowClassName;
            };
        }
    }
</script>
</head>

<body>
<ul>
    <li class="box">1</li>
    <li class="box green">2</li>
    <li class="box cc">3</li>
    <li class="box dd">4</li>
</ul>
</body>
</html>