public/404.html

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8" />
    <title>Single Page Apps for GitHub Pages</title>
    <script type="text/javascript">
      // Single Page Apps for GitHub Pages
      // https://github.com/rafrex/spa-github-pages
      // Copyright (c) 2016 Rafael Pedicini, licensed under the MIT License
      // ----------------------------------------------------------------------
      // This script takes the current url and converts the path and query
      // string into just a query string, and then redirects the browser
      // to the new url with only a query string and hash fragment,
      // e.g. http://www.foo.tld/one/two?a=b&c=d#qwe, becomes
      // http://www.foo.tld/?p=/one/two&q=a=b~and~c=d#qwe
      // Note: this 404.html file must be at least 512 bytes for it to work
      // with Internet Explorer (it is currently > 512 bytes)

      // If you're creating a Project Pages site and NOT using a custom domain,
      // then set segmentCount to 1 (enterprise users may need to set it to > 1).
      // This way the code will only replace the route part of the path, and not
      // the real directory in which the app resides, for example:
      // https://username.github.io/repo-name/one/two?a=b&c=d#qwe becomes
      // https://username.github.io/repo-name/?p=/one/two&q=a=b~and~c=d#qwe
      // Otherwise, leave segmentCount as 0.
      var segmentCount = 1;

      var l = window.location;
      l.replace(
        l.protocol +
          '//' +
          l.hostname +
          (l.port ? ':' + l.port : '') +
          l.pathname
            .split('/')
            .slice(0, 1 + segmentCount)
            .join('/') +
          '/?p=/' +
          l.pathname.slice(1).split('/').slice(segmentCount).join('/').replace(/&/g, '~and~') +
          (l.search ? '&q=' + l.search.slice(1).replace(/&/g, '~and~') : '') +
          l.hash
      );
    </script>
  </head>
  <body></body>
</html>