Nice work, fun little game and like the concept of how you reward for playing it.

Although I know this a little bit of fun to help you to learn Javascript, I'll share my feedback anyway :)

1. Why not use the keyboard to start the game rather than click, I use a laptop and use the touchpad and I can't tell you how many times I've died from clicking and not reacting fast enough. Maybe a countdown, but I assume that's more work than value.
2. It seems that if you double-click the send post to many times it can duplicate the comment, also if you successfully submit a comment and then click it again multiple times it will show the failure alert (informing you that you've already submitted) and then the successful alert.
3. Sometimes it will just pass over the dot when it should capture it, not repeatability to this, but just happens sometimes.
4. A change difficulty button would be nice, but obviously, I could type that into the URL.
5. A count of you score would also be cool, maybe play time too.
6. On hard the score seems to be doubled from what it actually posts in the comment.

Again, I know this is a game you've just thrown together, but thought it would be useful as some of these might be additional pieces you want to add in/resolve in the future.

Keep up the good work :D