清浮动，防止上下margin重叠（浏览器顶部空白崩溃）

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;}