<style type="text/css">
body{
margin:10px;
font-family: Arial;
font-size:13px;
}
#father{
background-color:#a0c8ff;
border:1px dashed #000000;
width:100%;
height:100%;
}
#block{
background-color: #fff0ac;
border:1p dashed #000000;
padding:10px;
position:absolute;
left:20px;
top:40px;
}
</style>
<div id="father"> <div id="block">absolute</div>

height:100%没有将容器撑开。
解决办法:
html,body{/*必须要html和body同时设置height:100%*/
margin:10px;
font-family: Arial;
font-size:13px;
height:100%;
}

原文:http://www.cnblogs.com/yee0403/p/5095228.html