<div id="redball"></div>

body { 
  margin: 0;
  background: #000;
  overflow: hidden;
  min-height: 100vh;
}
#redball { 
	background: red;
	width: 30vmin;
	height: 30vmin;
	border-radius: 50%;
}

var moveBall = document.getElementById('redball').animate([{
  transform: 'translateX(-20vw)'
}, {
  transform: 'translateX(100vw)'
}], {
  duration: 3000,
  iterations: Infinity,
  easing: 'ease'
});
moveBall.play();