Demo: CSS inline-size

<div class='container'>
  <div class='box'>Box 1</div>
  <div class='box'>Box 2</div>
  <div class='box'>Box 3</div>
</div>

.container {
  display: flex;
  gap: 10px;
}

.box {
  background-color: lightblue;
  inline-size: 150px;
  padding: 20px;
  text-align: center;
  border: 2px solid blue;
  height: 100px;
}

// No JavaScript required for this demo