Transitioning from GreenSock GSAP to Web Animations

We recently got a chance to work with the new Web Animations specification when building this year’s Google Santa Tracker. We normally use GreenSock GSAP for complex animation sequences so we were interested to see if the standards are catching up.

Please note that the Web Animations specification is very much a work in progress so things may have changed since this post was written.


There are many reasons to use GSAP — even Google recommends it. For complex scenarios, we need to be able to create animations purely in JavaScript. Preferably, they should run as efficiently as any CSS Animation or CSS Transition, and that’s exactly why there’s a new animation specification in town.

At the time of writing only Chrome and Opera has native support for Web Animations. There’s an official polyfill which works on modern versions of all major browsers.


Let’s compare some common animation tasks using both frameworks. The most basic animation is to tween an object’s properties from state A to state B.

Tip: to get the best performance possible we should avoid properties that cause a layout or paint, so that the GPU can carry out the work.

Basic GSAP tween

GSAP can tween any CSS property or DOM attribute, and uses a plugin architecture so you can load only the parts of the framework that you need.

The fastest way to get started is to use the to() method which automatically tweens the property from whatever it happens to be at the time the tween begins.

The x-property tells the CSS plugin to move the element using matrix() or translate() 2D transforms.

GSAP has most common easing functions built in and there are also from() and fromTo() functions for more control. You can force matrix3d() or translate3d() using the force3D flag to make the browser put that element onto its own compositor layer (use sparingly).

Note: GSAP even lets you define properties that are not generally tweenable and will apply the property for you like position:”absolute” and display:”none”.

Basic Web Animations tween

Web Animations also supports tweening any CSS property or DOM attribute, but you need to specify values for both the start keyframe and the end keyframe. You can provide custom easings using bezier curves (same as CSS easing).

This example is the most basic way of applying an animation directly on a DOM element.

One thing to watch out for is the fill mode. By default, Web Animations fill mode is set to none which means the element resets to whatever state it had before the animation. Setting it to forwards makes it keep the last keyframe state. Again, this behaves just like the CSS Animations counterpart.

Synchronizing tweens & Timeline controls

Both GSAP and Web Animations have timeline abstractions to help synchronize tweens. You can queue tweens on the timeline in sequence, in parallel and make them overlap.

GSAP timeline

The GSAP Timeline lets you add tweens, callbacks and labels with exact control of the timing offset between them. By default, tweens are added to the timeline in sequential order.

The timeline instance can be used to control all tweens by pausing, reversing, seeking and changing playback rate.

Web Animations timeline

Web Animations lets you create and combine AnimationSequences and AnimationGroups to synchronize the timing of individual tweens. They can be nested inside each other to create complex animations. A player must be created to play the outermost sequence/group.

The Web Animations player also supports pausing, reversing, seeking and changing playback rate.

Events and Callbacks

In one of the trickier scenes in this year’s Santa Tracker we had to animate random characters traveling on a conveyor belt. Half-way through their journey they would enter a magic closet that dressed them in a random color.

GSAP callback

The GSAP implementation of this use case is pretty straight forward thanks to the TimelineLite.add() function which can synchronize JavaScript callbacks alongside tweens.

GSAP also provides handy onStart, onUpdate, onComplete and onReverseComplete callbacks on both the Timeline and on individual Tweens.

Web Animations callback

This is where it gets a bit tricky. Web Animations only gives us a finish event on the player instance.

To schedule callbacks at a specific time during an animation, you need to manually wrap your callback in an animation and make sure it runs only once:

var myCallback = new Animation(elem, function(tf) {
  if (tf == null) return;
  console.log('Run callback here!');
}, { duration: 0, fill: 'forwards' });

The equivalent of the above GSAP animation sequence ends up looking like this with Web Animations:

You are probably wondering why we bind the finish event like that. The latest version of the spec. mentions that the event has been removed and that it should expose ready and finished promises instead. There’s an open issue for the polyfill tracking this.

Until this has been implemented everywhere, we need to do something like this to be future proof:

function finish(player, fn) {
  if (player.finished) {
  } else {
    player.onfinish = fn;  // or something to concat methods

If you need to do a lot of work with JavaScript callbacks, there’s an AnimationCallback class from Sam Thorogood that is worth checking out.

We would love to see more events and easier scheduling of callbacks added to the spec.


It turns out that Web Animations was a pretty good fit for the project, and we had a lot of fun working with it. It’s more mature than we thought, but it’s definitely a bit rough around the edges.

Things to watch out for:

  • The Web Animations syntax is very verbose compared to GSAP.
  • The way we found to schedule JavaScript callbacks with Web Animations feels like a hack.
  • It’s not clear if Web Animations players are cheap to create, or if we should be recycling them.
  • The finish event/promise is in flux.
  • It’s confusing to have 3 versions of the polyfill
    • We ran into different bugs using different polyfill versions.
    • It can be hard to keep track of which polyfill version is being used on your project. Polymer is currently shipped with the next-lite polyfill.
  • Web Animations gives you fine-grained control of the animation fill-mode. This is powerful but can get complicated as this example illustrates.

There’s a lot of advanced functionality in GSAP that probably won’t be covered by Web Animations, but at least we will have a native choice going forward. And GSAP should be able to optimize its engine even further by taking animations entirely off the main JavaScript thread.


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