<div class="flex-container">
  <div class="flex-item order3">
    order: 3<br>
    DOM 순서는 첫번째
  </div>
  <div class="flex-item order0">
    order: 0
  </div>
  <div class="flex-item order1">
    order: 1
  </div>
  <div class="flex-item order2">
    order: 2
  </div>
</div>

.flex-container {
  display: flex;
  margin-bottom: 20px;
  padding: 10px;
  background-color: #ccc;
}

.flex-item {
  margin: 10px;
  padding: 10px;
  background-color: yellow;
}

.order0 {
  order: 0;
}

.order1 {
  order: 1;
}

.order2 {
  order: 2;
}

.order3 {
  order: 3;
  color: white;
  background-color: green;
}