OME [1] is a RTMP server for sub-second WebRTP streaming. Profiles allow for specific encodes if needed. I have a repository hosting an exemplary configuration and a frontend using their player [2].

Another server is SRS [3], but IMO it is more difficult to use, lacks features and the delay is about 1-3s — it requires far less bandwidth though.

[1] https://github.com/AirenSoft/OvenMediaEngine [2] https://github.com/hashworks/simple-ome [3] https://github.com/ossrs/srs