Highlighter already integrates with any web site or article on a web site. If I can highlight something and leave a comment, and see the comments my contacts make, it’s like Disqus, but way way better and doesn’t require any centralizing force or permission.
Discussion
I’m imagining a browser extension that is a Nostr client itself. It’s always open when I’m browsing the web or reading articles on the web and can see notes that have referenced that article/product/etc. if there are too many, I can filter by only seeing my friends.