为什么main里面要加一个相对定位 position:relative???
.top{height:100px;background:#cfcfcf;}
.main{background:red;}
.left{width:200px;height:600px;background:blue; position:absolute; top:100px; left:0px;}
.right{background:#7bc69f;width:100%; margin:0 0 0 205px; height:600px;}
.foot{background:#e37411;}
