@keyframes fadeIn {
  from { opacity: 0; }
  to { opacity: 1; }
}

/* Usage */
.element {
  animation-name: fadeIn;
  animation-duration: 500ms;
  animation-timing-function: ease;
  animation-delay: 0ms;
  animation-iteration-count: 1;
  animation-direction: normal;
  animation-fill-mode: none;

  /* Shorthand */
  animation: fadeIn 500ms ease 0ms 1 normal none;
}

/* With prefers-reduced-motion */
@media (prefers-reduced-motion: reduce) {
  .element {
    animation: none;
  }
}