The animationstart event is fired when a CSS Animation has started. If there is an animation-delay, this event will fire once the delay period has expired. A negative delay will cause the event to fire with an elapsedTime equal to the absolute value of the delay (and, correspondingly, the animation will begin playing at that time index into the sequence).

The original target for this event is the Element that had the animation applied. You can listen for this event on the Window interface to handle it in the capture or bubbling phases. For full details on this event please see the page on HTMLElement: animationstart.

This listens for the animationstart event and logs a message when it is fired:

window.addEventListener('animationstart', () => {
  console.log('Animation started');
});

The same, but using onanimationstart:

window.onanimationstart = () => {
  console.log('Animation started');
};

See a live example of this event.

• CSS Animations
• Using CSS Animations
• AnimationEvent
• Related events: animationend, animationiteration, animationcancel
• This event on Document targets: animationstart
• This event on HTMLElement targets: animationstart