That is a use of SRV records [1], however it was not accepted into the HTTP protocol specification. I bring it up every time there is a new protocol version but I am too lazy to write an RFC addendum for it and hope that someone else will. Existing protocols may not be modified in this manor once ratified. Maybe HTTP/4.0? /s

Some applications use SRV records for load balancing. Many VoIP and video conferencing apps do this. There is a better list on Wikipedia.

    _service._proto.name. TTL class SRV priority weight port target.

[1] - https://en.wikipedia.org/wiki/SRV_record