flex属性设置详解:http://www.cnblogs.com/lvmylife/p/7670149.html
CSS实现垂直居中:https://www.cnblogs.com/lvmylife/p/5453844.html
关于CSS中display的32种写法:https://baijiahao.baidu.com/s?id=1599614277825293572&wfr=spider&for=pc
元素垂直对齐：http://www.w3school.com.cn/cssref/pr_pos_vertical-align.asp
display:table:https://www.cnblogs.com/yuxiaobai/p/9290491.html