The onanimationend property of the GlobalEventHandlers mixin is the event handler for processing animationend events.
The animationend event fires when a CSS animation reaches the end of its active period (which is calculated as (animation-duration * animation-iteration-count) + animation-delay).
var animEndHandler = target.onanimationend;
target.onanimationend = Function
A Function to be called when an animationend event occurs indicating that a CSS animation has ended on the target, where the target object is an HTML element (HTMLElement), document (Document), or window (Window). The function receives as input a single parameter: an AnimationEvent object describing the event which occurred.