纯 CSS 水平垂直居中一个元素

方法1

position: absolute;
left: 0;
top: 0;
bottom: 0;
right: 0;
margin: auto;

方法2

position: absolute;
left: 50%;
top: 50%;
transform: translate3d(-50%, -50%, 0);

方法3

position: absolute;
left: 50%;
top: 50%;
margin-left: -50px;
margin-top: -50px;
width: 100px;
height: 100px;