#loader,
#loader:before {
  height: 5px;
  margin: 0;
  width: 100%;
}

#loader {
  background-color: darkgray;
  display: flex;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 40%;
  visibility: hidden;
}

#loader:before {
  background-color: black;
  content: "";
  animation: running-progress 2s cubic-bezier(0.4, 0, 0.2, 1) infinite;
}

@keyframes running-progress {
  0% {
    margin-left: 0;
    margin-right: 100%;
  }

  50% {
    margin-left: 25%;
    margin-right: 0;
  }

  100% {
    margin-left: 100%;
    margin-right: 0;
  }
}