如何设置背景图片填充整个屏幕

        background: url(../assets/img/showBg.png) no-repeat top center;
        background-size: cover;
        background-attachment: fixed;
        width: 100%;
        height: 100%;
        position: fixed;
        top: 0;
        left: 0;
        right: 0;
        bottom: 0;