[nathanstitt]

Also not affiliated but my open-source tinycld uses docx as the backend storage for its text package. Supports _most_ of the features (including comments and suggestions) but is still very young. It has a golang backend that reads/writes docx and translates to YJS that the editor reads for multi-user access. Has web/iOS/Android support.

I found docx to be a very well documented format and a surprisingly good fit for this.

https://tinycld.org has a live demo