This is cool!

A possible extension to HTMX would be to allow this kind of offloading to pure JS functions instead of requiring hacky intercepts.

You would still have a clear separation of responsibilities between frontend rendering (by the browser only) and application logic (which only generates HTML as output).

I've done the same thing with vanilla JS in a service worker. I did it with my HTMF library -- similar to HTMX -- but not production ready and based on forms rather than being able to put your attributes everywhere. I'll have to create a simple to do app with HTMX that shows this same functionality. The Rust one is cool as you can do it with 150kB library. But I can do the same thing with 10kB with just simple JS.