<header>
  <h1>Title</h1>
</header>
<main>
  Adding <code>flex:1</code> to <code>&lt;main&gt;</code> will force it to take the remaining space
</main>
<footer>
  Credits
</footer>

html {
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
}
body {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}
header {
  height: 60px;
  background: #ccc;
}
main {
  flex-grow:1;
}
footer {
  height: 60px;
  background: #ccc;
}