原文地址：
https://blog.csdn.net/liuhailiuhai12/article/details/55252283
1. .parent{
width:400px;
height:500px;
display:flex;
justify-content: center;
align-items: center;
}
2.單行文字垂直居中
line-height等於父類的height
3.display：table和table-cell
.parent {
display: table;
width: 300px;
height: 300px;
text-align: center;
}
.son {
display: table-cell;
height: 200px;
background-color: yellow;
vertical-align: middle;
}
原文地址：https://www.cnblogs.com/chen-cong/p/8076442.html