I remember seeing something similar a long time ago. As I can remember you were able to scribble on a website, and everyone else using that plugin or website (I don't remember if it was a software or site) would see what you have drawn.
Didn’t really work out for them: https://variety.com/2021/digital/news/genius-sold-medialab-w...
http://alumni.media.mit.edu/~orit/utok.html
I imagine nowadays all the notes would just be like any online comments section: fighting and name-calling about anything and everything
This may be a good application for decentralized storage with free browser extensions that would circumvent much of the legal threads - although I imagine lawsuits could and would still be filed against Chrome, Firefox etc for enabling such extensions, even if they were free.
If memory serves me, something similar was very briefly a feature of Internet Explorer.
Except with the IE version, everyone else using IE (which was everyone, even Mac users) could also see everyone else's notes.
The project was discontinued over concerns about intellectual property and defacing other people's web sites.
It was a long time ago (as illustrated by the reference to IE for Mac ), so I may have some details confused.