JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>
<aside class="log"></aside>

CSS

div {
    background-color: blue;
    width: 100px;
    height: 100px;
    transition: background-color 2s;
}

JavaScript

var div = document.getElementsByTagName("div")[0];
div.addEventListener('transitionend', function() {
    document.querySelector('aside.log').textContent += 'transitionend';
});              
div.style.backgroundColor = 'green';
// Part-way through change to something non-interpolable
window.setTimeout(function() {
    div.style.backgroundColor = 'none';
}, 1000);