清除浮动
.clearfix:after{
content:”.”;
display:block;
height:0;
clear:both;
visibility:hidden;
}
清除margin溢出问题
.overflow:before{
content: ".";
display: block;
height: 0;
visibility: hidden;
}
原文：http://www.cnblogs.com/duanyue/p/7307878.html