p {
  animation: translate-down 1s infinite alternate linear;
}
@keyframes margin-down {
  from {
    margin-top: 0;
  }
  to {
    margin-top: 30px;
  }
}
@keyframes margin-down {
  from {
    transform: translateY(0);
  }
  to {
    transform: translateY(300px);
  }
}