/* div在屏幕中居中对齐的方法一：*/
        margin: auto;
        position: absolute;
        top: 0;
        left: 0;
        right: 0;
        bottom: 0;

 /* div在屏幕中居中对齐的方法二：*/
        position: absolute;
        left: 50%;
        top: 50%;
        transform: translate(-50%,-50%);

/* div在屏幕中居中对齐的方法三：*/
       display: flex;
       justify-content: center;
       align-items: center;