JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container" class="container"></div>

CSS

.container {
    height: 200px;
    width: 200px;
    background-color: salmon;
    display: none;
    border-radius: 5px;
    opacity: 0;
    transition: opacity 2s ease-in-out;
}

JavaScript

var container = document.querySelector("#container");

container.style.display = "block";

//this is not working
//container.style.opacity = 1;


//this is working
setTimeout(function() {
   container.style.opacity = 1;
}, 0);