A border around the playfield would help - I keep trying to draw lines that extend slightly outside the browser window, and inadvertently do things like grabbing a bookmark from the bookmarks bar above.
Yeah, that would help a lot. Also might be possible to see where the ball comes back in a little earlier then.
Another little helper would be to draw the level-start-lines not completely up to the border. Because often you can then still route around them all to reach otherwise unsolvable levels.