Alert messages are a terrible way to handle any kind of in-game notifications. They block the whole page, not just the game window, and you have to close them, so if it weren't for your menu screen, I wouldn't even be able to write this review without an interruption every three seconds.
The game itself is quite simple (so is the graphics), but it can provide some entertainment for a while. If you wanted to work on the game further, you could for example add few different models of trees (now there's just one model of everything), try to make the obstacles blend in the background more (because they don't do at all now) and possibly improve animations. Also, for some reason you begin with -1 Deaths, that's probably a bug. Good Luck.