Code snippet

<div class="card"></div>

.card {
  background: linear-gradient(
    90deg,
    hsl(139, 33%, 28%) 25%,
    hsl(139, 29%, 17%) 37%,
    hsl(139, 33%, 28%) 63%
  );
  background-size: 400% 100%;
  animation: skeleton 1.2s infinite;
}

@keyframes skeleton {
  from {
    background-position: 100%;
  }

  to {
    background-position: 0;
  }
}