A long time ago, there was a W3C standard for this called web annotations². It never really caught on, possibly because people and organizations were expected to host their own annotation servers. NCSA Mosaic had the feature back in 1993. Check out this post from Marc Andreesen on the www-talk list: http://1997.webhistory.org/www.lists/www-talk.1993q2/0416.ht... A rather obscure browser by the W3C called Annotea also…
It's focused on several related specs, like making timestamped copies of pages (like archive.org), selecting ranges in pages, making selections that are resilient to page changes, and maybe more I'm not aware of.
They have a GitHub repo that has some other work as well: https://github.com/apache/incubator-annotator/