今天学到一套很强大css样式，据相关人士透露，只要加入这些元素，万物皆可实现居中
1.居中
display: flex; justify-content: center; align-items: center; flex-direction: row;
2.左右留空，平均分配居中
display: flex; align-items: center; flex-direction: row; justify-content: space-around;
3.左右不留空，平均分配居中
display: flex; align-items: center; flex-direction: row; justify-content: space-between;
4.平均分配居中
display: flex; align-items: center; flex-direction: row; justify-content: space-evenly;
5.垂直居中
display: flex; align-items: center; flex-direction: column; justify-content: center;
