1. position: absolute;
top:0;
left:0;
right: 0;
bottom:0;
margin:auto;
2. position: absolute;
top:50%;
left:50%;
margin:-75px;
3. position: absolute;
top:50%;
left:50%;
transform: translate(-50%,-50%);
4. display: flex;
justify-content: center;
align-items: center;
5. 父 display: flex;
justify-content: center;
子 align-self: center;
