How to align the last row to the grid with CSS Flexbox?

To align the last row to the grid with CSS Flexbox, we use the :after selector.

For instance, we write

.grid {
  display: flex;
  flex-flow: row wrap;
  justify-content: space-between;
}

.grid::after {
  content: "";
  flex: auto;
}

to add the flex: auto; to the part of the page after the element with class grid to align the last row to the grid.