JSFiddle - React, Tailwind, and code Playground

HTML

<div id="triangle"></div>
<button id="doit">Do it!</button>

CSS

#triangle {
    width: 0;
    height: 0;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-top: 100px solid red;
    transition: all .5s ease;
}
#triangle.mod {
    border-left-width: 100px;
}

JavaScript

var triangle = document.querySelector('#triangle'),
    button = document.querySelector('#doit');

button.addEventListener('click', function(e) {
    e.preventDefault();
    triangle.classList.add('mod');
}, false);