JanusXR

 JanusXR  is an ostensibly-decentralized, self-hosted, open-source platform. See https://janusxr.org/.

The company that was behind it has folded as of 2019, but one of the developers has continued working on it as a spare-time project. An example instance is accessible at https://web.janusxr.org.

It takes design cues from VRML, by allowing rooms to be built up from assets which are arranged using XML markup.