[html] Clear all fields in a form upon going back with browser back button

Modern browsers implement something known as back-forward cache (BFCache). When you hit back/forward button the actual page is not reloaded (and the scripts are never re-run).

If you have to do something in case of user hitting back/forward keys - listen for BFCache pageshow and pagehide events:

window.addEventListener("pageshow", () => {
  // update hidden input field
});

See more details for Gecko and WebKit implementations.