404.html

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Page Not Found</title>
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <style>
    html {
      color: #333;
      display: table;
      font-family: sans-serif;
      height: 100%;
      text-align: center;
      width: 100%;
      line-height: 1.2;
      margin: 0;
    }
    body {
      display: table-cell;
      vertical-align: middle;
      margin: 2em auto;
    }
    p {
      margin: 16px auto;
      width: 280px;
    }
  </style>
</head>
<body>
  <h1>Page Not Found</h1>
  <p>The page you were trying<br>to view does not exist</p>
  <p><a onclick="Back()" href="#">Go back</a> / <a href="/">Go home</a></p>
</body>
<script type="text/javascript">
  function Back() {
    window.history.back();
  }
</script>
</html>