Pretty fun. Suggestion to increase the timer with each successful submission as a reward. Also, when the player highlights all tiles on the board the game becomes a dead end and must be restarted.
Whenever you highlight one or more tiles, ideally you should be able to click outside of the number grid to clear the highlights. Do let me know if that didn't work for you. I think I can call attention to the game play instructions on the website a bit better as well though.
Thanks for the feedback!