W3Schools Tryit Editor

<!DOCTYPE html>
<html>
<head>
<style>
.item1 { grid-area: 1 / 1 / 2 / 2; }
.item2 { grid-area: 1 / 2 / 2 / 3; }
.item3 { grid-area: 1 / 3 / 2 / 4; }
.item4 { grid-area: 2 / 1 / 3 / 2; }
.item5 { grid-area: 2 / 2 / 3 / 3; }
.item6 { grid-area: 2 / 3 / 3 / 4; }

.grid-container {
  display: grid;
  grid-auto-columns: 50px;
  grid-gap: 10px;
  background-color: #2196F3;
  padding: 10px;
}

.grid-container > div {
  background-color: rgba(255, 255, 255, 0.8);
  text-align: center;
  padding: 20px 0;
  font-size: 30px;
}
</style>
</head>
<body>

<h1>The grid-auto-columns Property</h1>
<p>Use the <em>grid-auto-columns</em> property to set a default size (width) for all columns.</p>
<p>Set the size to 50 pixels per column:</p>

<p><strong>Note:</strong> The grid-template-columns property overrides this property.</p>

</body>
</html>