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.
Hypothesis is one that has been interesting to use for web article annotation. It lets you save quotes and comments on articles https://web.hypothes.is/
A great product, it has support for practically any browser thanks to the bookmarklet and chrom(ium) extension [0]. I mostly use it on PDFs (e.g. papers). I definitely recommend it.
A big problem from what I recall were legal ones. E.g., owners of a website did not fancy random comments graffitied over their content. I believe a few lawsuits were filed. Also spam problems arose. This caused the makers of such software to hobble functionality, making the resulting product less interesting and so these products were not successful.
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.
Back in the early 00s there was a browser plugin called Third Voice that did this. I seem to remember people got concerned that it would lead to "graffiti-ing" of websites. I don't remember what happened to it.
StumbleUpon had something like this, you could go to a webpage and comment on it and others could see your comments. I stopped using it as it was a massive time waster/procrastination tool. But was pretty cool.
