①:给父盒子display:flex;
Align-items:center;
Justify-contont:center;
②:父盒子position:relative;
子元素position:absolute;
Left:0;
Right:0;
Top:0;
Bottom:0;
Margin:auto;
③:父盒子:position:relative;
子元素:position: absolute;
Left:50%;
Top:50%;
Transform:translate(-50%,-50%);