Marko Denic:

It’s the eternal battle between day and night, good and bad. Written in JavaScript with some HTML & CSS in one index.html. Feel free to reuse the code and create your own version.

Completely and utterly hypnotic. And unending - unless, as sometimes happens, the two opposing “balls” get locked reciprocally wiping out a single brick. At a guess, it’s impossible for it to ever end, because the square with the ball in can’t be wiped out by the opposing ball.