<div id="document">
  <main>
    <h1>Everything apart from the footer goes here</h1>
    <p>Add more text here, to see how the footer responds!</p>
  </main>
  <footer>
    <h1>The footer goes here</h1>
  </footer>
</div>

#document { 
    height: 100vh;
    display: flex;
    flex-direction: column;
}

main {
  flex: auto;
}

footer {
    flex-shrink: 0;
}

/* Other styling elements, that are not necessary for the example */

* {
  margin: 0;
  font-family: Candara;
}

h1, p {
  padding: 20px;
}

footer {
  color: white;
  background: url(https://images.unsplash.com/photo-1550795598-717619d32900?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=676&q=80);
  background-position: center; 
  background-repeat: no-repeat;
  background-size: cover;
}

footer > h1 {
  text-shadow: 1px 1px 4px #00000080;
}