#container {
  display: flex;                  /* establish flex container */
  flex-direction: row;            /* default value; can be omitted */
  flex-wrap: nowrap;              /* default value; can be omitted */
  justify-content: space-between; /* switched from default (flex-start, see below) */
  background-color: lightyellow;
}
#container > div {
  width: 100px;
  height: 100px;
  border: 2px dashed red;
}