You won't be able to do it natively.

But you could be clever and use web sockets (if you need 2 way comms) to continuously save the form data (just like this website does!) - or just use a vanilla Ajax call `onkeyup` - then save it in the PHP session, to be echo'ed out later.

Or a more straightfoward pluggable approach would be to use a library like [jQuery Save as you Type](https://github.com/BenGriffiths/jquery-save-as-you-type)