其实是class添加了after属性
.icon-chrome:after{content:"忠实读者";position:absolute;background-color:rgba(0,0,0,.84);color:#fff;font-size:12px;border-radius:2px;top:-25px;padding:3px;width:60px;text-align:center;left:-20px;display:none}
.icon-chrome:hover:after{display:block}