Demo: CSS grid-auto-flow

<div class='grid-container'>
  <div class='grid-item'>1</div>
  <div class='grid-item'>2</div>
  <div class='grid-item'>3</div>
  <div class='grid-item'>4</div>
  <div class='grid-item'>5</div>
  <div class='grid-item'>6</div>
</div>

.grid-container {
  display: grid;
  grid-template-columns: repeat(3, 100px);
  grid-auto-flow: column;
  gap: 10px;
}

.grid-item {
  background-color: lightblue;
  border: 2px solid blue;
  padding: 20px;
  text-align: center;
  font-size: 20px;
}

// No JavaScript required for this demo