body{font-family: "Helvetica Neue", Helvetica, Arial, "PingFang SC", "Hiragino Sans GB", "Heiti SC", "Microsoft YaHei", "WenQuanYi Micro Hei", sans-serif;}
header{line-height:80px;height: 80px;background:#fff;}
header a{text-align: left;background:#fff;}
header button{height:80px;position: relative;}
header a:hover{background:#fff;
    }
header a:before,header a:after,header a:active{
    color: #1890ff;
    content: "";
    width: 0;
    height: 5px;
    background: #1890ff;
    position: absolute;
    top: 100%;
    left: 50%;
    transition: all .5s;
}

header a:hover:after{/*动画效果是从中间向右延伸至50%的宽度*/
    left: 0%;
    width: 100%;

}