清浮动
父级添加类别！
.clearfix{zoom:1;//兼容ie6,7}
.clearfix:after{
content:".";
display: "block";
height: "0";
clear: both;
visibility: hidden;
}
防止上下margin重叠，也可以清除浮动塌陷（推荐）
.clearfix:after,
.clearfix:before{
content:" ";
display: table;
}
.clearfix:after{clear: both;}
.clear{ clear:both;}