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

body {
  margin: 0;
  background: #000;
  overflow: hidden;
  min-height: 100vh;
}

#redball {
  background: red;
  width: 30vmin;
  height: 30vmin;
  border-radius: 50%;
  animation: moveBall 3s infinite;
}

@keyframes moveBall {
  from {
    transform: translateX(-20vw);
  }
  to {
    transform: translateX(100vw);
  }
}