body {
  background:
    linear-gradient(to bottom, #ff2d2d 0, #ff2d2d 2px, #2a2a2a 2px, #121212) repeat-y calc(50% - 50px) 0 / 50px 200px,
    linear-gradient(to bottom, #ff2d2d 0, #ff2d2d 2px, #2a2a2a 2px, #121212) repeat-y calc(50% + 50px) 0 / 50px 200px
    #121212;
  animation: anim 11s linear infinite;
}

@keyframes anim {
  to {
    background-position-y: calc(-200px * 4), calc(-200px * 5);
  }
}