Custom 404 page
A request whose path matches no redirect rule and no file in the deploy is answered 404. If the deploy has a 404.html at its root, that page is served with the 404 status. Otherwise a Kamakiri not-found page is served.
There is no single-page-app fallback. An unmatched path is never answered with index.html and a 200.
Reference assets by absolute path
The same 404.html is served for every missing path, so write the URLs it references as absolute paths: /style.css, not style.css. A relative URL resolves against the missing path, and the stylesheet or image fails to load.
<!doctype html>
<meta charset="utf-8">
<title>Not found</title>
<link rel="stylesheet" href="/style.css">
<h1>Not found</h1>