listener = function() {
          if (self.isInViewport(self.parentEl, 1)) {
            self.play();
            window.removeEventListener('scroll', listener);
          }
        }
Trigger to start of the animation.
Depending on the `start` value, a different script
will be applied.

If the `start` value is not valid, an error will be thrown.
Even if technically, this is impossible.