JSFiddle - React, Tailwind, and code Playground

HTML

<div style="border: 1px solid red;" class="popup"></div>

CSS

.popup
    {
        width : 300px !important;
        height: 300px;
    }

JavaScript

setTimeout(function() {
    var css = document.createElement("style");
    css.type = "text/css";
    css.innerHTML = ".popup { width : 500px !important; }";
    document.body.appendChild(css)
}, 5000);