The NCSA httpd 0.4 release in 1993 announced support for Group Annotations, which was basically this. So all of you "I had this idea four years ago" people need to get in line behind Marc Andreesen, who had this idea, dear god, 16 years ago.
It never really took off--I think even though httpd supported annotations, I don't think Mosaic itself ever really did.
"Probably the best operating system in the world is the [operating system] made for the PDP-11 by Bell Laboratories." - Ted Nelson, October 1977