HTML animationstart DOM Event: Reference and Examples

When someone interacts with a webpage, the browser sends events to the relevant JavaScript code. The animationstart event happens when a CSS animation begins.

When animationstart happens

A CSS animation begins.

You can listen for it using addEventListener(). That keeps event handling separate from HTML attributes such as onclick and works well with modern code organization.

A small working example

<style>
    @keyframes demoPulse { from { opacity: 1; } to { opacity: .4; } }
    .animated-box { animation: demoPulse 2s 2 alternate; }
</style>
<div id="demo-target" class="animated-box">Animate this box with CSS</div>
<p id="event-status" role="status">Waiting for animationstart...</p>
<script>
    const eventTarget = document.getElementById("demo-target");
    const eventStatus = document.getElementById("event-status");

    if (eventTarget) {
        eventTarget.addEventListener("animationstart", (eventDetails) => {
            eventStatus.textContent = "animationstart event received";
            console.log(eventDetails.type, eventDetails.target);
        });
    }
</script>

Reading the code

The browser listens for animationstart on the relevant target. When the event occurs, the callback receives eventDetails, updates the status message and logs the event name and target in Developer Tools.

Apply an animation class after the page loads.

The CSS rule above makes the animation or transition possible. Without a real animated property, a listener alone cannot make these events fire.

Try it in your browser

Open the page, show the Console panel, and follow this test: Apply an animation class after the page loads. Then change one thing in the example and observe whether the event still fires.

Common Beginner Mistakes

• Expecting animationstart to fire without its required user action or browser condition.
• Apply an animation class after the page loads.
• Adding repeated event listeners without considering cleanup or unwanted side effects.

Key Takeaways

• animationstart reports a CSS animation begins.
• addEventListener("animationstart", ...) listens for the event on the appropriate target.
• Apply an animation class after the page loads.