水平垂直居中

.game-box {
    width: 480px;
    height: 850px;
    position: relative;
}
#btnStart{
    width: 240px;
    height: 80px;
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%，-50%);
    //子绝父相 left top设置成50%   横纵都偏移自身的50%
}

.game-box {
    width: 480px;
    height: 850px;
    display: flex;
    justify-content: center;
    align-items: center;
}
//弹性布局 为父元素设置display:flex