An event handler for the animationstart event. This event is sent when a CSS Animation starts to play.

A Function to be called when an animationstart event occurs indicating that a CSS animation has begun 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.

Leaving out some bits of the CSS that don't matter for the discussion here, let's take a look at the styles for the box that we're animating. First is the box itself. We set its size, position, color, and layout. Note that there's nothing there about animation. That's because we don't want the box to start animating right away. We'll add the animation style later to start animating the box.

#box {
  width: var(--boxwidth);
  height: var(--boxwidth);
  left: 0;
  top: 0;
  border: 1px solid #7788FF;
  margin: 0;
  position: relative;
  background-color: #2233FF;
  display: flex;
  justify-content: center;
}

The animation sequence is described next. First, the "slideAnimation" class, which establishes the animation that will cause the box to move over the course of five seconds, one time, using the "slideBox" keyframe set. The keyframes are defined next; they describe an animation which causes the box to migrate from the top-left corner of the container to the bottom-right corner.

.slideAnimation {
  animation: 5s ease-in-out 0s 1 slideBox;
}

@keyframes slideBox {
  from {
    left:0;
    top:0;
  }
  to {
    left:calc(100% - var(--boxwidth));
    top:calc(100% - var(--boxwidth))
  }
}

Since the CSS describes the animation but doesn't connect it to the box, we'll need some JavaScript code to do that. We'll get to that shortly.

Before we get to the animation code, we define a function which logs information to a box on the user's screen. We'll use this to show information about the events we receive. Note the use of AnimationEvent.animationName and AnimationEvent.elapsedTime to get information about the event which occurred.

function log(msg, event) {
  let logBox = document.getElementById("log");

  logBox.innerHTML += msg;

  if (event) {
    logBox.innerHTML += " <code>"+ event.animationName +
        "</code> at time " + event.elapsedTime.toFixed(2) +
        " seconds.";
  }

  logBox.innerHTML += "\n";
};

Then we set up the event handlers for the animationstart and animationend events:

let box = document.getElementById("box");

box.onanimationstart = function(event) {
  log("Animation started", event);
}

box.onanimationend = function(event) {
  log("Animation stopped", event);
};

Finally, we set up a handler for a click on the button that runs the animation:

document.getElementById("play").addEventListener("click", function(event) {
  document.getElementById("box").className = "slideAnimation";
  event.target.style.display = "none";
}, false);

This sets the class of the box we want to animate to the class that contains the animation description, then hides the play button because this example will only run the animation once. For information about why, and how to support running an animation more than once, see Run an animation again in CSS Animations tips and tricks.

Assembled together, you get this:

• The animationstart event this event handler is triggered by.
• AnimationEvent