If you are truly offline, then of course this won't work.

Though, it is certainly possible to cache pages since most sites do not need persistent state - it's just ephemeral things like toggling a button, linking an input box and another caption, etc...

Though you could run the datastar js sdk from a service worker and if you've synced necessary state to browser storage, you could have your backend there.

Also, for slow connections, compression is very powerful - especially over an sse stream where there's redundant info being sent. Another comment here has links to immense demos and the compression ratios are north of 90%.

Also, slow internet is often also associated with slow devices, which cannot handle the bloat of React, css-in-js etc

Datastar VASTLY simplifies everything while sacrificing essentially nothing in the way of functionality.