CSS fixed-width block elements are centered horizontally and vertically.

1、use absolute + marign auto layout.

absolute margin auto.

2、use absolute + negative margin layout.

negative margin.

3、use table-cell layout.

table-cell.

4、use absolute + transform layout.

transform-in.

5、use float + negative marign layout.

float-in.

6、use inline-block + empty element layout.

inline-block.

7、use flex layout.

flex.