Sizing tracks in CSS Grid

Sizing tracks in CSS Grid by Wes Bos

HTML

<link rel="stylesheet" href="https://cdn.rawgit.com/wesbos/css-grid/master/assets/style.css">
<div class="container">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">Wes Bos Is Cool</div>
  <div class="item">4</div>
  <div class="item">5</div>
  <div class="item">6</div>
  <div class="item">7</div>
  <div class="item">8</div>
  <div class="item">9</div>
  <div class="item">10</div>
  <div class="item">11</div>
  <div class="item">12</div>
  <div class="item">13</div>
  <div class="item">14</div>
  <div class="item">15</div>
</div>

CSS

.container {
  display: grid;
  height: 600px;
  grid-gap: 20px;
  border: 10px solid var(--yellow);
  grid-template-columns: auto 1fr auto 1fr;
}