CSS_圣杯

/*
 * 两端固定,中间自适应!!为圣杯
 */
<style type="text/css">
.site{height:500px;padding:0 50px;}
.middle{float:left;width:100%;height:100%;background:#CCC;}
.left{float:left;width:50px;height:100%;background:#CCF;margin-left:-100%;position:relative;left:-50px;_left:50px;}
.right{float:left;width:50px;height:100%;background:#CFC;margin-left:-50px;position:relative;left:50px;}
</style>
<div class="site">
    <div class="middle"></div>
    <div class="left"></div>
    <div class="right"></div>
</div>