#flashHR {
    border: none;
    height: 2px;
    background: linear-gradient(to right, transparent 0%, #000000 50%, transparent 0%);
    background-size: 200% 100%;
    animation: flashOnce 1s forwards; /* 1s duration, only once */
  }

  @keyframes flashOnce {
    0% {
      background-position: -100% 0;
    }

    100% {
      background-position: 100% 0;
    }

  }