@keyframes bounce {
    0%, 20%, 50%, 80%, 100% {
      transform: translateY(0);
    }
    40% {
      transform: translateY(-20px);
    }
    60% {
      transform: translateY(-15px);
    }
  }

  #btn {
    animation: bounce 2s infinite;
  }

/* ---------------------- */

@keyframes bounce {
    0%, 20%, 50%, 80%, 100% {
      transform: translateY(0);
    }
    40% {
      transform: translateY(-20px);
    }
    60% {
      transform: translateY(-15px);
    }
  }

  .btn {
    animation: bounce 2.5s infinite;
  }