From RFC 7230:

```A server MAY send a Content-Length header field in a response to a
   HEAD request (Section 4.3.2 of [RFC7231]); a server MUST NOT send
   Content-Length in such a response unless its field-value equals the
   decimal number of octets that would have been sent in the payload
   body of a response if the same request had used the GET method.```

```A server MAY send a Content-Length header field in a 304 (Not
   Modified) response to a conditional GET request (Section 4.1 of
   [RFC7232]); a server MUST NOT send Content-Length in such a response
   unless its field-value equals the decimal number of octets that would
   have been sent in the payload body of a 200 (OK) response to the same
   request.```

```Transfer-Encoding MAY be sent in a response to a HEAD request or in a
   304 (Not Modified) response (Section 4.1 of [RFC7232]) to a GET
   request, neither of which includes a message body, to indicate that
   the origin server would have applied a transfer coding to the message
   body if the request had been an unconditional GET. This indication
   is not required, however, because any recipient on the response chain
   (including the origin server) can remove transfer codings when they
   are not needed.```