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                <div class="wrapper">
  <div class="stage">
    <span id="prop-name"></span>
    <div id="subject">div</div>
  <button id="start">Start Animation</button>
  <div id="output"></div>


                #subject {
  text-transform: lowercase; /* start */

@keyframes go {
  to {
    text-transform: uppercase; /* end */

/* you probably don't need to change anything below */

*, *::before, *::after {
  box-sizing: border-box;

html, body {
  height: 100%;

body {
  display: grid;
  align-items: center;
  justify-items: center;
  margin: 0;
  color: #f8f9fa;
  background-color: #343a40;
  font: 1.25rem/1 'Roboto', sans-serif;

.wrapper {
  display: grid;
  grid-template-columns: 1fr 1fr;
  grid-template-rows: 1fr min-content;
    'stage output'
    'start output';
  gap: 20px;
  width: min(1200px, 100vw);
  height: min(400px, 100vh);
  padding: 20px;

.stage {
  grid-area: stage;
  display: grid;
  align-items: center;
  justify-items: center;
  position: relative;
  padding: 10px;
  background-color: #f8f9fa;
  overflow: hidden;

#prop-name {
  position: absolute;
  top: 0;
  left: 0;
  padding: 10px;
  background-color: #495057;

/* essentially #subject, but with lower specificity */
[id="subject"] {
  display: grid;
  align-items: center;
  justify-items: center;
  width: 80px;
  height: 80px;
  border: 2px solid #ced4da;
  color: #495057;

.animate {
  animation: go 1s linear forwards;

#start {
  grid-area: start;
  padding: 10px;
  border: 1px solid #f8f9fa;
  color: #fff;
  background-color: #a61e4d;
  font: inherit;
  cursor: pointer;
  -webkit-appearance: none;
  appearance: none;

#start:disabled {
  cursor: progress;

#output {
  grid-area: output;
  padding: 10px;
  color: #fff;
  background-color: #212529;
  font: 1rem/1.5 'Roboto Mono', monospace;
  white-space: pre;
  overflow: auto;


To experiment with a different CSS property:
1. Update the lines marked with "start" and "end" in the CSS.
2. Update `animatedPropName` in the JS below.
3. Run the experiment!

Note: the layout CSS of this codepen itself can bork some animations. For example, animating `float` does nothing because the subject div is positioned via `grid`.

const animatedPropName = 'text-transform';

/* you probably don't need to change anything below */

const start = document.getElementById('start');
const subject = document.getElementById('subject');
const output = document.getElementById('output');
const propName = document.getElementById('prop-name');

start.addEventListener('click', startExperiment);
subject.addEventListener('animationstart', startAnimation);
subject.addEventListener('animationend', endAnimation);

propName.innerText = animatedPropName;

let isAnimating = false;
let startTime;

function startExperiment() {
  // change button
  start.disabled = true;
  start.innerText = 'Animating...';
  // clear previous experiment
  output.innerText = '';
  // begin animation

function startAnimation() {
  isAnimating = true;
  startTime =;

function endAnimation() {
  // wrap things up
  isAnimating = false;
  // change button
  start.disabled = false;
  start.innerText = 'Restart Animation';

function update() {
  if (isAnimating) {
    const elapsedTime = - startTime;
    addSnapshot(elapsedTime + 'ms');

function addSnapshot(time) {
  const styles = window.getComputedStyle(subject);
  const propValue = styles.getPropertyValue(animatedPropName);
  output.innerText += time.padStart(5) + ': ' + propValue + '\n';
  // scroll output to bottom
  output.scrollTop = output.scrollHeight;