I was today years old when I learned something new about the CSS order property. I didn't learn about the existence of the property today, mind you. I just learned about a constriction which, in my opinion, makes it a little bit less usable. You see... when you want to order a flex or grid item, you have to order all of them. You can't just order one of them. Let me show you what I mean.

According to its formal definition on MDN, order's inital value is 0 (zero).

When you order an item, it will be visually ordered according to its value compared to its siblings's order value. Any item with the same order value will be visually ordered by its place in the DOM. Have a look at this example:

<style>
  .flex-container {
    display: flex;
  }
</style>
<div class="flex-container">
  <div class="flex-item">Item 1</div>
  <!-- order: 0 // default -->
  <div class="flex-item">Item 2</div>
  <!-- order: 0 // default -->
  <div class="flex-item" style="order: 1">Item 3</div>
  <!-- order: 1 // explicitly assigned -->
</div>

<!-- Expected visual result -->
Item 1 Item 3 Item 2

<!-- Actual visual result -->
Item 1 Item 2 Item 3

You would expect Item 2 and Item 3 to be visually swapped. However, they are not swapped, despite Item 3 having an order: 1. Sort of like zero-index Arrays, 1 comes after 0. If you want to actually order one item, you have to order all of them. This is true for both flex and grid.

To order any element, you have to order all of them. This is not a big problem, as you usually generate lists of items with a loop (like v-for in Vue or *ngFor in Angular or Array.prototype.map in JSX). You can easily retrieve the current iteration index and use that for order. It's just weird to me that it is necessary. I would expect order to have a default value of auto, making the visual order default to the DOM order.

So, to make sure that Item 3 is visually ordered before Item 2, you have to explicitly set order: 2 on Item 2 (or any value higher than 1).

<div class="flex-container">
  <div class="flex-item" style="order: 1">Item 1</div>
  <div class="flex-item" style="order: 2">Item 2</div>
  <div class="flex-item" style="order: 1">Item 3</div>
</div>

<!-- Visual result -->
Item 1 Item 3 Item 2

No big deal, but something to keep in mind when you want to use the order property.