CSS Keyframes support is pretty good these days:
.fade-in {
opacity: 1;
animation-name: fadeInOpacity;
animation-iteration-count: 1;
animation-timing-function: ease-in;
animation-duration: 2s;
}
@keyframes fadeInOpacity {
0% {
opacity: 0;
}
100% {
opacity: 1;
}
}
<h1 class="fade-in">Fade Me Down Scotty</h1>