I just read the article #[2]
I see you are referring to building Nostr reading client, text reading this functionality is 100% possible on web browsers.
The only important caveat that I can think of is that there would be to be some guard rails in place so readers can only highlight valid text. As adding HTML elements to the DOM could break things if not handled properly.
This shouldn't be a problem with pure text though. But multi-section or multi-paragraph highlighting could have some unique (but minor) challenges.
I'm a writer and software engineer so this sounds like a great idea!