Pen Settings

CSS Base

Vendor Prefixing

Add External Stylesheets/Pens

Any URL's added here will be added as <link>s in order, and before the CSS in the editor. If you link to another Pen, it will include the CSS from that Pen. If the preprocessor matches, it will attempt to combine them before processing.

+ add another resource

You're using npm packages, so we've auto-selected Babel for you here, which we require to process imports and make it all work. If you need to use a different JavaScript preprocessor, remove the packages in the npm tab.

Add External Scripts/Pens

Any URL's added here will be added as <script>s in order, and run before the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

+ add another resource

Use npm Packages

We can make npm packages available for you to use in your JavaScript. We use webpack to prepare them and make them available to import. We'll also process your JavaScript with Babel.

⚠️ This feature can only be used by logged in users.

Code Indentation


Save Automatically?

If active, Pens will autosave every 30 seconds after being saved once.

Auto-Updating Preview

If enabled, the preview panel updates automatically as you code. If disabled, use the "Run" button to update.

HTML Settings

Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

              <h1>Detect Transition Start</h1>
<p>There is no method for detecting a css transition start in JavaScript. This demo uses the transitionEnd event in a sneaky way to detect transition start.</p>
<p>It requires wrapping the transitionEnd event with a custom event 'transition-end'. This alone is useful for dealing with browser prefixes.</p>
<p>But this solution is not yet ideal. It also requires some specific css with an invisible 0.00001s transition to detect the start. It doesn't matter what property you transition but it must change. The search for a better solution continues...<p>
<a id="hover-me">Hover me</a>
<div id="transition-status"></div>
<p>This could be useful in cases similar to how the window resize event is used. If the height or width of a container is changing with css transitions, you may wish to update something during the transition. Not just when it ends.<p>
    transition: opacity 0.00001s linear, color 3s linear; /* must be 0.00001s. 0s did not work :( */
    background: #CCC;
    padding: 10px;
    border-radius: 10px;
    border: solid 1px;
    display: inline-block;    
  opacity: .999999; /* Doesn't matter what property but it must change */
  color: red;
  cursor: pointer;

  margin: 1em 0;
// Detect which transition event (from modernizer)
var whichTransitionEvent = function(){
  var t;
  var el = document.createElement('fakeelement');
  var transitions = {

  for(t in transitions){
	if([t] !== undefined ){
	  return transitions[t];

var transitionEvent = whichTransitionEvent();

	// Set-up transition-start custom event
	if (window.CustomEvent) {
	  var transitionStartEvent = new CustomEvent('transition-start',{'bubbles': true, 'cancelable': true});
	  var transitionEndEvent = new CustomEvent('transition-end',{'bubbles': true, 'cancelable': true});
	} else {
	  var transitionStartEvent = document.createEvent('CustomEvent');
	  var transitionEndEvent = document.createEvent('CustomEvent');
	  transitionStartEvent.initCustomEvent('transition-start', true, true);
	  transitionEndEvent.initCustomEvent('transition-end', true, true);
	document.body.addEventListener(transitionEvent, function(evnt) {
		// Guard detect and trigger transition start here
    var target = || evnt.srcElement;
    // FF does not report the exact time here.
		if(evnt.elapsedTime <= 0.00001){;
		} else {;

var statusElm = document.getElementById("transition-status");
var hoverElm = document.getElementById("hover-me");

hoverElm.addEventListener('transition-start', function() {
	var d = new Date();
	statusElm.innerHTML = d.getHours() +':'+ d.getMinutes() +':'+ d.getSeconds() +':'+ d.getMilliseconds() +' Transition has started.';

hoverElm.addEventListener('transition-end', function() {
	var d = new Date();
    statusElm.innerHTML = d.getHours() +':'+ d.getMinutes() +':'+ d.getSeconds() +':'+ d.getMilliseconds() +' Transition has ended.';
🕑 One or more of the npm packages you are using needs to be built. You're the first person to ever need it! We're building it right now and your preview will start updating again when it's ready.