Oh! Sorry, this was not supposed to happen. That menu is the system menu over which I have no control but even then RESET in theory should just return to main menu, not erase save. There is, however, an issue with certain browsers (Chrome) where savegames are not recorded. When you revisit the page, does it say "Save OK" on the top left in the main screen?
I downloaded the offline version... and after loading the game again, my save is still there!!! ^_^ sorry for the confusion - I feel like when I used the "reset game" option it took me to the title screen, but I didn't see the "press z to continue save game". It looks like closing and opening fixed it I guess?