gistfile1.css

@navigation-enter(0.5s) {
  html {
    background-color: transparent;
    animation: fade-in 0.5s;
  }
}

@keyframes fade-in {
  from { opacity: 0; }
  to { opacity: 1; }
}

gistfile2.css

@media (navigation-enter: 0.5s) {
  html {
    background-color: transparent;
    animation: fade-in 0.5s;
  }
}

@keyframes fade-in {
  from { opacity: 0; }
  to { opacity: 1; }
}