<!doctype html>
<meta charset="utf-8">
<style>
.constr{ width:800px;}
.constr > div {width:50%; height:100px; margin:auto; background-color:#0CC;}
#d2{position:absolute; left:0; right:0;}
</style>
<div class="constr">
<div id="d1"></div>
<div id="d2"></div>
</div>
楼上的同学试试
