Comment by graypegg
Your browser (if you're using one of the "usual ones") doesn't really do much with the response's status code if it doesn't match a few specific ones for redirecting/caching/protocol shenanigans.
Anything in the 4XX range is going to be treated as just a regular ol' response, just like 404. (You could serve an entire site with all responses set to status=404, and be fine... other than probably never getting any cache hits) If you don't include a body in the response, the browser might sub in it's own error page, but it will just communicate that the user agent made a bad request.
204 has weird behavior in Safari and Firefox for example. Entering a URL returning 204 in the URL bar will not change the URL bar to it, leaving its contents to whatever was there before. Similarly if you click on it it would not actually navigate to the page.
URL to test: https://httpbin.org/status/204