ホーリーグレイルレイアウトのCSS Grid実装

<div class="grid-container">
  <div class="item-1">header</div>
  <div class="item-2">nav</div>
  <div class="item-3">main</div>
  <div class="item-4">aside</div>
  <div class="item-5">footer</div>
</div>

.grid-container {
  display: grid;
  grid-template-columns: repeat(5, 1fr);
  grid-template-rows: repeat(5, 1fr);
  gap: 8px;
  width: 100%;
  height: 500px;
}

.item-1 { grid-area: 1 / 1 / 2 / 6; }
.item-2 { grid-area: 2 / 1 / 5 / 2; }
.item-3 { grid-area: 2 / 2 / 5 / 5; }
.item-4 { grid-area: 2 / 5 / 5 / 6; }
.item-5 { grid-area: 5 / 1 / 6 / 6; }

@media (max-width: 768px) {
  .grid-container {
    grid-template-columns: 1fr;
    grid-template-rows: auto;
    height: auto;
  }
  .grid-container > * {
    grid-area: auto;
  }
}