三列布局：左右固定，中间自适应宽度
偷下懒，css就不单独写出来了···
<div style="position:absolute; left:0; top:0;width:200px; height:400px; background:#ccc;">asd</div>
<div style="background:#f60; margin:0 300px 0 200px; height:200px;"></div>
<div style="position:absolute; right:0; top:0;width:300px; height:400px; background:#000;"></div>
