Currently, waitress 204 response looks like this:

HTTP/1.1 204 No Content
Allow: GET, DELETE, HEAD, OPTIONS
Connection: close
Date: Tue, 06 Jun 2017 07:43:55 GMT
Server: waitress
Transfer-Encoding: chunked
Vary: Accept
X-Frame-Options: SAMEORIGIN

Transfer-Encoding violates RFC 7230:

https://tools.ietf.org/html/rfc7230#section-3.3.1

A server MUST NOT send a Transfer-Encoding header field in any response with a status code of 1xx (Informational) or 204 (No Content).

https://tools.ietf.org/html/rfc7230#section-3.3.2

A server MUST NOT send a Content-Length header field in any response with a status code of 1xx (Informational) or 204 (No Content).

In practice, such behavior breaks some clients, such as requests (if server is behind Google Cloud Load Balancer).