References

Beginner-friendly references for web development, with live, editable examples.

The HTML onanimationstart event

Event All modern browsers Updated
Quick answer

The HTML onanimationstart attribute runs JavaScript when a CSS animation starts. It is an inline handler for the animationstart event; in modern code prefer addEventListener('animationstart', …).

Overview

The onanimationstart event attribute runs JavaScript when a CSS animation starts. In JavaScript the event itself is named animationstart — drop the on prefix when you call addEventListener.

It is a CSS animation event. The handler receives an AnimationEvent with the animationName and elapsedTime, letting you run code at specific points in a CSS animation defined with @keyframes.

You can wire this up with the inline onanimationstart HTML attribute, but the modern, recommended approach is element.addEventListener('animationstart', handler) in JavaScript. That keeps behavior out of your markup, lets you attach several handlers to the same event, and makes them easy to remove. The inline attribute is fine for quick demos.

Syntax

<element onanimationstart="handler()">…</element>

element.addEventListener('animationstart', handler);

Best practices

  • Prefer element.addEventListener('animationstart', handler) over the inline onanimationstart attribute — it separates behavior from markup and allows multiple handlers.
  • Use it to run JavaScript at a precise point in a CSS animation without a timer.
  • Check event.animationName when an element has more than one animation.
  • Respect prefers-reduced-motion for the animations these events track.

Frequently asked questions

What is the onanimationstart event?
It runs JavaScript when a CSS animation starts. In JavaScript the event is named animationstart.
How do I run code when a CSS animation finishes?
Listen for the animationend event on the animated element.
How do I know which animation triggered the event?
Read event.animationName from the AnimationEvent.
Should I use the onanimationstart attribute or addEventListener?
Prefer addEventListener('animationstart', …) in JavaScript. The inline onanimationstart attribute works but mixes behavior into the markup and allows only one handler per element.
onanimationiteration onauxclick