<!-- float+负margin（两侧定宽，中间自适应） -->
<div class="m-box">
<div class="center">中</div>
</div>
<div class="left">左</div>
<div class="right">右</div>
html,body{margin:0; padding: 0;}
.left,.right,.m-box .center { height: 200px; line-height:200px; text-align:center; }
.m-box { float:left; width: 100%;}
.m-box .center {margin:0 200px;background-color: #C7DAFC;}
.left{float: left; width: 200px; margin-left: -100%; background-color: #FAD7F0;}
.right {margin-left: -200px;width: 200px;float: left;background-color: #FAD7F0;}