CSS制作hover下划线动画

.demo1{
            position: relative;
            text-decoration: none;
            font-size: 20px;
            color: #333;
        }
        .demo1:before{
            content: "";
            position: absolute;
            left: 50%;
            bottom: -2px;
            width:;
            height: 2px;
            background: #4285f4;
            transition: all .3s;
        }
        .demo1:hover:before{
            width: 100%;
            left:;
            right:;
        }