<body>
  <div class="content">
   <header>
    ヘッダ
   </header>
   <main>
    コンテンツ
   </main>
  </div>
  <footer>
    フッタ
  </footer>
</body>

html {
  height: 100%;
}
body {
  margin: 0;
  padding: 0;
  min-height: 100%;
  display: grid;
  grid-template-rows: 1fr auto;
}
header {
  width: 100%;
  background: #ccdd33;
}
footer {
  grid-row-start: 2;
  grid-row-end: 3;
  width: 100%;
  background: #44aa33;
}

External CSS

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External JavaScript

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