src/404.html

<!DOCTYPE html>
<html>
<head>
  <link rel="preload" href="./styles/css/styles.css" as="style"/>
  <link rel="stylesheet" href="./styles/css/styles.css"/>
  <link rel="preload" href="./libs/require.js" as="script"/>
  <!-- favicon -->
  <link rel="apple-touch-icon" sizes="180x180" href="./styles/favicon/apple-touch-icon.png"/>
  <link rel="icon" type="image/png" sizes="32x32" href="./styles/favicon/favicon-32x32.png"/>
  <link rel="icon" type="image/png" sizes="16x16" href="./styles/favicon/favicon-16x16.png"/>
  <link rel="manifest" href="./styles/favicon/site.webmanifest"/>
  <link rel="mask-icon" href="./styles/favicon/safari-pinned-tab.svg" color="#5bbad5"/>
  <meta name="apple-mobile-web-app-title" content="NASA ADS"/>
  <meta name="application-name" content="NASA ADS"/>
  <meta name="msapplication-TileColor" content="#ffc40d"/>
  <meta name="theme-color" content="#ffffff"/>
  <!-- /favicon -->
  <script src="./libs/require.js"></script>
  <!-- Google Tag Manager -->
  <script>(function (w, d, s, l, i) {
    w[l] = w[l] || [];
    w[l].push({
      'gtm.start':
        new Date().getTime(), event: 'gtm.js',
    });
    var f = d.getElementsByTagName(s)[0],
      j = d.createElement(s), dl = l != 'dataLayer' ? '&l=' + l : '';
    j.async = true;
    j.src =
      'https://www.googletagmanager.com/gtm.js?id=' + i + dl;
    f.parentNode.insertBefore(j, f);
  })(window, document, 'script', 'dataLayer', 'GTM-NT2453N');</script>
  <!-- End Google Tag Manager -->
  <script>
    require.config({
      paths: {
        underscore: [
          'https://unpkg.com/lodash@2.4.2/dist/lodash.compat.min',
          'libs/lodash',
        ],
        analytics: 'js/components/analytics',
        jquery: [
          'https://unpkg.com/jquery@2.2.4/dist/jquery.min',
          'libs/jquery',
        ],
      },
    });
    require(['analytics'], function (analytics) {
      analytics('send', 'virtual_page_view', '/404');
      analytics(
        'send',
        'event',
        'error',
        'automatic_redirection',
        '404.html',
      );
    });
  </script>
</head>
<body>
<div
  style="display: flex; flex-direction: column; min-height: screen; min-width: screen; justify-content: center; align-items: center;"
>
  <h1>Bumblebee failed to load. Please try again.</h1>

  <h2>
    <a href="javascript:history.back()">Go Back</a>
  </h2>
</div>
</body>
</html>