JSFiddle - React, Tailwind, and code Playground

HTML

<div class="fadeIn">bye</div>

<div class="unfaded">hi</div>

CSS

body {
    font-size: 3em;
}

div {
    opacity: 0;
}

div.fadeIn {
    -webkit-animation: fadeIn 2s forwards;
    animation: fadeIn 2s forwards;
}

@keyframes fadeIn {
    0% {opacity: 0;}
    100% {opacity: 1;}
}

@-webkit-keyframes fadeIn {
    0% {opacity: 0;}
    100% {opacity: 1;}
}

JavaScript

setTimeout(function() {
  document.querySelector('.unfaded').className = 'fadeIn';
}, 2000);