块级元素垂直居中

1. 父盒子弹性布局

display: flex;
align-items: center;
justify-content: center;

2. 父元素 relative，子元素 absolute

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

3. 父元素 relative，子元素 absolute

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