Yeah, founded a startup based on this idea in 2006 or so, called WeBrowse. The technology wasn't quite ready although I got pretty far with a prototype based on DOMMutationListener and XHR.

Pretty clearly something like this needs to exist, and it will probably not be nearly as useful as it's inventors think it will be. Screen-sharing is more general, and the niche for low-entropy real-time synchronization of browser state feels pretty small right now. But who knows? Heck, I thought the iPad was going to be a giant flop.

Cool! I do agree this concept is under-utilized. Best of luck!

(Heads up: if you shall ever venture to patent this tech, I think I'll be able to present compelling prior art from like 1996. This was also a proxy-based re-writing mechanism for co-browsing and from your description it does sound kinda similar. It also happened to use HTML frames.)

Mozilla together js did this a while back.. https://togetherjs.com

I remember customer support doing co-browsing back in 2008. Hopefully the mechanisms have gotten a little less hacky over the years.