There have been many such systems. I built one in the early 2000s and tried to raise money around it, to no avail. At the time there were several commercial competitors, including one well-funded venture called ThirdVoice. Wikipedia has some information on the history of the genre: https://en.wikipedia.org/wiki/Web_annotation .
There where a bunch of us trying it. We created a corporate version for web site annotation and collaboration. We lost our best chance for funding just as the market cratered.
Having seen several more since then I don't think the market is there for it. People still rather print/screen capture/write an email/annotate PDF versions, about changes in websites rather than use an annotation tool on the page it seems.
> People still rather print/screen capture/write an email/annotate PDF versions, about changes in websites rather than use an annotation tool on the page it seems.
The reason seems fairly obvious - printing/screen capture/PDFs are off-line. Permanent and under user's control. Internet based annotations are inherently unstable - one way today, different tomorrow, completely gone next week. I always found the concept a great thing for sharing thoughts with people, but a terrible one for personal notes.