Oh, we did all that. I even wrote my PhD dissertation on it (2001). Back then, there were quite a few research projects working on what I then termed ‘Web Augmentation’. You could do it serverside, in special proxies, or in the browser through extensions or scripting. The earliest browser based system I know of that allowed users to insert links and annotations into existing Web pages was DHM/WWW, developed by yours truly in 1997 – we even won best paper for it at Hypertext 1997. This was a specialised case of what was then known as Open Hypermedia, i.e., a hypermedia system that could integrate (more or less) arbitrary documents and systems through links and other structures.

Using a more general architecture, it was possible to add many kinds of structures on top of existing Web pages collaboratively. Navigational or spatial hypermedia, guided tours, etc. Link bases were stored outside of the linked resources, so that you could have different sets of structures applied to a corpus of documents.

There were also quite a few commercial attempts in this space. However, all, commercial and research alike, ultimately failed, and I believe that there were several reasons behind this: Web site owners disliked that others could annotate their pages; shared link bases often became polluted; bookmarks were ‘good enough’ for most users; maintaining link anchor consistency in (increasingly dynamic) Web pages became more and more difficult (despite our best efforts); and while I remain enthused about the notion of the MEMEX scholar carefully annotating their corpus of literature, I don’t think the appeal is all that widespread.

Still, there are remnants in some systems – e.g., I enjoy to see that others have highlighted the same passage as I in a Kindle book.