The level design means it's not impossible to get yourself into a fail state, where the two squares are cut off from each other and can't be reunited, and the game doesn't really offer you a way out of this except refreshing the browser to start over.
Also, unless there's a reason that W/Up is being used for something else, it might make sense to bind jump to those keys as well? The controls are a little fiddly when you're trying to move both at once.