Nice game. I was first a bit confused what I was supposed to do, but then found that exploding multiple enemies at once was really satisfying.
However, consider adding additional action button besides SPACE, as pressing it always activates my browser's scroll bar. I could play the game on other browser or change options, but I'm saying this as something to consider.
Background music is an obvious note to times when our parents got angry for playing those "beepy ringtones" from our phones. (Or that was the first feeling I got from it). Also, the game loaded much faster compared to other games with similar size/amount of content, which is always a good thing.