It is strange because, for me, I take the file and put it on a window of safari , then I keep it in the favorites.
I play it without problem, and when I want to quit (to keep the position) I go to the menu and then I can go to another page of the web or quit safari.
When I come back to the game a day after I click the favorite submachine, and I retrieve the last position.
But your solution must also be ok... ;)