取消浮动

clear

.box{
    width:100px;
    height:100px;
    background-color:red;
    clear:left;
}
清除左浮动

.box{
    width:100px;
height:100px;
background-color:red;
clear:right;
}
清除右浮动

both 可以清除对它影响最大的浮动
可以解决高度塌陷

.clearfix:after

.clearfix:after{
    content:' ';
    display:hidden;
    clear:both;
}

如果塌陷就加上clearfix