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 name of the CSS property associated with the transition.
The amount of time the animation has been running, in seconds, as of the time the event fired, excluding any time the animation was paused. This value should be 0 unless animation-delay is a negative value, in which case the value will be (-1 * animation-delay), and the animation will begin to play back at that time offset into the animation sequence.