Scoring. You win points for hitting balls and lose them (but not so many) for missing. Faster moving balls carry more points (in relation to the square of the speed). The size, shape and colour of the balls do not matter (in this version). Can you keep your score positive?
Bad eyesight? Then, keep the mouse pointer outside the playing area!
Keystrokes being ignored? Sometimes keyboard focus gets lost, even though the browser window is highlighted. Move the mouse pointer into (or out from) the playing area to force a regain of focus ...