clear:both ,IE8 ！！ foot ！！！ IE6,7 ！！
clear:both (IE5 )！！
<html>
<head>
<title> New Document </title>
<style type ="text/css">
#head{width:100%; height:20px; background:#789;}
#left{float:left; width:200px; height:300px; background:#823;}
#right{float:right; width:200px; height:300px; background:#399;}
#foot{clear:both; width:100%; height:20px; background:#789;}
</style>
</head>
<body>
<div id="head">head</div>
<div id="left">left</div>
<div id="right">right</div>
<div id="foot">footer</div>
</body>
</html>