JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://www.w3schools.com/w3css/4/w3.css">


<div class="w3-container">
  <h2>Animated Modal</h2>

  <button onclick="document.getElementById('id01').style.display='block'" class="w3-button w3-black">Fade In Modal</button>

  <div id="id01" class="w3-modal w3-animate-opacity">
    <div class="w3-modal-content w3-card-4">
      <header class="w3-container w3-teal"> 
        <span  onclick="document.getElementById('id01').classList.add('w3-animate-show');"
        class="w3-button w3-large w3-display-topright">&times;</span>
        <h2>Modal Header</h2>
      </header>
      <div class="w3-container">
        <p>Some text..</p>
        <p>Some text..</p>
      </div>
      <footer class="w3-container w3-teal">
        <p>Modal Footer</p>
      </footer>
    </div>
  </div>
</div>

CSS

.w3-animate-opacity{animation:opac 0.8s}@keyframes opac{from{opacity:0} to{opacity:1}}
    
.w3-animate-show{animation: show 0.8s; animation-fill-mode:forwards;}@keyframes show{0%{opacity:1}   100%{opacity:0}}