解决子元素浮动之后，父元素不被撑开的问题

1.父元素加class --  clearfix:after {content: ".";display: block;height: 0;clear: both;visibility: hidden;}     * html .clearfix {height: 1%;}

2.子元素后加<div style="clear:both"></div> 或者给父元素加属性overflow:hidden;

3.父元素加属性display:table