div {
  border: 1px solid black;
}

#a {
  width: 300px;
  height: 200px;
  display: box;
  box-orient: horizontal;
}

#b {
  box-flex: 1;
}

#c {
  box-flex: 1;
}

#d {
  display: box;
  box-orient: vertical;
  box-flex: 2;
}

#e {
  box-flex: 1;
  box-ordinal-group: 2;
}

#f {
  box-flex: 1;
}