That is indeed part of the design. I could've easily implemented a button to restart the level, or a button to restart the game, but the choice for F5 and it reloading the page is deliberate. Restarting the game should feel like restarting the game. I actually wrote a custom handler for F5 just to be able to give that warning/confirmation dialogue box. It's all yet more hints to how to solve the last puzzle and should make total sense once you solved it. Did you manage to finish the game?