清除浮动

1.最常用的

.clear {clear:both;overflow:hidden;height:0;border:none;}

2.如果父元素允许overflow为hidden

.clearfix {zoom:1;overflow:hidden;}

3.如果不考虑ie6

.fix {zoom : 1}
.fix:after {content:'.';display:block;visibility:hidden;clear:both;line-height:0;}