the effect is as follows:
there are three rows like this, and each flex item occupies 33.3333% of the principal axis space, so I divided two horizontal flex containers as the previous vertical flex item. Now the problem is how to arrange the first two flex items in line 2 (if the width of the flex item is not limited, I think it cannot be divisible). Now I add an empty flex item after the second line, and then enlarge it with flex:1. But with an empty div, ask the boss for advice on a better way.