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…
Hypothesis (Hypothes.is) [2] is active [3] and awesome. They are involved in the Web Annotation standardization efforts.