app/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>