为何第二个模块在下面？
<!dcotype html>
<html>
<head>
<meta charset="utf-8">
<title>三列布局</title>
<style type="text/css">
body{margin:0;paddig:0}
.yi{width:300px;height:400px;background:red;postiton:absolute;left:0;top:0;}
.er{height:400px;background:blue;margin:0 300px 0 310px;}
.san{width:300px;height:400px;background:yellow;position:absolute;right:0;top:0;}
</style>
<body>
<div class="yi">一</div>
<div class="er">二</div>
<div class="san">三</div>
</body>
</html>
