<div class="m-box">
<div class="cont">右</div>
</div>
<div class="m-asider">左</div>
html,body{margin:0; padding: 0;}
.m-box .cont,.m-asider { height: 200px; line-height:200px; text-align:center; }
.m-box { float:left; width: 100%;}
.m-box .cont {margin-left:210px;background-color: #ccc;}
.m-asider {float: left; width: 200px; margin-left: -100%; background-color: #ccc;}