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Animation Worklet Explainer


AnimationWorklet is a new primitive for creating scroll-linked and other high performance procedural animations on the web. It is being incubated here as part of the CSS Houdini task force, and if successful will be transferred to that task force for full standardization.


Scripted effects (written in response to requestAnimationFrame or async onscroll events) are rich but are subject to main thread jankiness. On the other hand, accelerated CSS transitions and animations can be fast (for a subset of accelerated properties) but are not rich enough to enable many common use cases and currently have no way to access scroll offset and other user input. This is why scripted effects are still very popular for implementing common effects such as hidey-bars, parallax, position:sticky, and etc. We believe (and others agree) that there is a need for a new primitive for creating fast and rich visual effects with the ability to respond to user input such as scroll.

This document proposes an API to create custom animations that execute inside an isolated execution environment, worklet. It aims to be compatible with Web Animations and uses existing constructs as much as possible. We believe this API hits a sweet spot in balancing among performance, richness, and rationality for addressing our key use cases.

This design supersedes our CompositorWorker proposal.

Motivating Use Cases

  • Scroll-linked effects:

  • Animations with custom timing functions (particularly those that are not calculable a priori)

    • Spring timing function (demo)
  • Location tracking and positioning:

    • Position: sticky
  • Procedural animation of multiple elements in sync:

    • Efficient Expando (demo, more info)
    • Compositing growing / shrinking box with border (using 9 patch)
  • Animating scroll offsets:

    • Having multiple scrollers scroll in sync e.g. diff viewer keeping old/new in sync when you scroll either (demo)
    • Implementing smooth scroll animations (e.g., custom physic based fling curves)

Note: Demos work best in the latest Chrome Canary with the experimental web platform features enabled (--enable-experimental-web-platform-features flag) otherwise they fallback to using main thread rAF to emulate the behaviour.

Key Concepts

Animation Worklet Global Scope

A worklet global scope that is created by Animation Worklet. Note that Animation Worklet creates multiple such scopes and uses them to execute user defined effects.


WorkletAnimation is a subclass of Animation that can be used to create an custom animation effect that runs inside a standalone animation worklet scope. A worklet animation has a corresponding animator instance in a animation worklet scope which is responsible to drive its keyframe effects. Here are the key differences compared to a regular web animation:

  • AnimationId should match a specific animator class registered in the animation worklet scope.
  • WorkletAnimation may have multiple timelines (including ScrollTimelines).
  • WorkletAnimation may have a custom properties bag that can be cloned and provided to animator constructor when it is being instantiated.

Note that worklet animations expose same API surface as other web animations and thus they may be created, played, paused, inspected, and generally controlled from main document scope. Here is how various methods roughly translate:

  • cancel(): cancels the animation and the corresponding animator instance is removed.
  • play(): starts the animation and the corresponding animator instance may get its animate function called periodically as a result of changes in its timelines.
  • pause(): pauses the animation and the corresponding animator instance no longer receives animate calls.
  • finish(): invokes finish on the corresponding animator instance.
  • reverse() or mutating playbackRate: invokes playbackRateChanged on the corresponding animator instance.


ScrollTimeline is a concept introduced in scroll-linked animation proposal. It defines an animation timeline whose time value depends on scroll position of a scroll container. ScrollTimeline can be used an an input timeline for worklet animations and it is the intended mechanisms to give read access to scroll position.


GroupEffect is a concept introduced in Web Animation Level 2 specification. It provides a way to group multiple effects in a tree structure. GroupEffect can be used as the output for worklet animations. It makes it possible for worklet animation to drive effects spanning multiple elements.

TODO: At the moment, GroupEffect only supports just two different scheduling models (i.e., parallel, sequence). These models governs how the group effect time is translated to its children effect times by modifying the child effect start time. AnimationWorklet allows a much more flexible scheduling model by making it possible to to set children effect's local time directly. In other words we allow arbitrary start time for child effects. This is something that needs to be added to level 2 spec.

Multiple Timelines

Unlike typical animations, worklet animations can be attached to multiple timelines. This is necessary to implement key usecases where the effect needs to smoothly animate across different timelines (e.g., scroll and wall clock).

Primary Timeline

The first timeline is considered the primary timeline. The only purpose of the primary timeline is to make integration with existing web animation machinery easier, in particular the primary timeline time will be used anywhere the web animation API needs to expose a time value, for example in event timeline time, or event current time.

TODO: We are considering API designs that can make it possible for an animation to observe multiple timelines but only gets activated on a (dynamic) subset of them. This ensures we can be more efficient when updating the animation.

Animator Migration

The animators are not guaranteed to run in the same global scope (or underlying thread) for their lifetime duration. For example, a user agents is free to initially run the animator on main thread but later decide to migrate it off main thread to get certain performance optimizations. To allow worklet animators to keep their state across migrations, the API provides the following lifetime hooks:

// in document scope
new WorkletAnimation('animation-with-local-state', [], [], {value: 1});
registerAnimator('animation-with-local-state', class {
  constructor(options, state) {
    // |options| may be either:
    //  - The user provided options bag passed into the WorkletAnimation constructor on first initialization i.e, {value: 1}.
    //  - The object returned by |destroy| after each migration i.e. {value: 42}.
    this.options_ = options;
    this.state_ = state || {value: Math.random()};

  animate(timelines, effect) {
    this.state_.value += 0.1;
    effect.localTime = this.state_.value;

  destroy() {
    // Invoked before each migration attempts.
    // The returned object must be structure clonable and will be passed to constructor to help
    // animator restore its state after migration to the new scope.
    return this.state_;


TODO: Add gifs that visualize these effects

Hidey Bar

An example of header effect where a header is moved with scroll and as soon as finger is lifted it animates fully to close or open position depending on its current position.

<div id='scrollingContainer'>
  <div id='header'>Some header</div>

await CSS.animationWorklet.addModule('hidey-bar-animator.js');

const scrollTimeline = new ScrollTimeline({
  scrollSource: $scrollingContainer,
  orientation: 'block',
  timeRange: 100
const documentTimeline = document.timeline;

const animation = new WorkletAnimation('hidey-bar',
  new KeyframeEffect($header,
                      [{transform: 'translateX(100px)'}, {transform: 'translateX(0px)'}],
                      {duration: 100, iterations: 1, fill: 'both' })
  {scrollTimeline, documentTimeline},


registerAnimator('hidey-bar', class {

  constructor(options) {
    this.scrollTimeline_ = options.scrollTimeline;
    this.documentTimeline_ = options.documentTimeline;

  animate(currentTime, effect) {
    const scroll = this.scrollTimeline_.currentTime;  // [0, 100]
    const time = this.documentTimeline_.currentTime;

    // **TODO**: use a hypothetical 'phase' property on timeline as a way to detect when user is no
    // longer actively scrolling. This is a reasonable thing to have on scroll timeline but we can
    // fallback to using a timeout based approach as well.
    const activelyScrolling = this.scrollTimeline_.phase == 'active';

    let localTime;
    if (activelyScrolling) {
      this.startTime_ = undefined;
      localTime = scroll;
    } else {
      this.startTime_ = this.startTime_ || time;
      // Decide on close/open direction depending on how far we have scrolled the header
      // This can even do more sophisticated animation curve by computing the scroll velocity and
      // using it.
      this.direction_ = scroll >= 50 ? +1 : -1;
      localTime = this.direction_ * (time - this.startTime_);

    // Drive the output effects by setting its local time.
    effect.localTime = localTime;

Twitter Header

An example of twitter profile header effect where two elements (avatar, and header) are updated in sync with scroll offset.

<div id='scrollingContainer'>
  <div id='header' style='height: 150px'></div>
  <div id='avatar'><img></div>

await CSS.animationWorklet.addModule('twitter-header-animator.js');
const animation = new WorkletAnimation('twitter-header',
  [new KeyframeEffect($avatar,  /* scales down as we scroll up */
                      [{transform: 'scale(1)'}, {transform: 'scale(0.5)'}],
                      {duration: 1000, iterations: 1}),
    new KeyframeEffect($header, /* loses transparency as we scroll up */
                      [{opacity: 0}, {opacity: 0.8}],
                      {duration: 1000, iterations: 1})],
    new ScrollTimeline({
      scrollSource: $scrollingContainer,
      timeRange: 1000,
      orientation: 'block',
      startScrollOffset: 0,
      endScrollOffset: $header.clientHeight}),


registerAnimator('twitter-header', class {
  constructor(options) {
    this.timing_ = new CubicBezier('ease-out');

  clamp(value, min, max) {
    return Math.min(Math.max(value, min), max);

  animate(currentTime, effect) {
    const scroll = currentTime;  // [0, 1]

    // Drive the output group effect by setting its children local times.
    effect.children[0].localTime = scroll;
    // Can control the child effects individually
    effect.children[1].localTime = this.timing_(this.clamp(scroll, 0, 1));


.parallax {
    position: fixed;
    top: 0;
    left: 0;
    opacity: 0.5;
<div id='scrollingContainer'>
  <div id="slow" class="parallax"></div>
  <div id="fast" class="parallax"></div>

await CSS.animationWorklet.addModule('parallax-animator.js');
const scrollTimeline = new ScrollTimeline({
  scrollSource: $scrollingContainer,
  orientation: 'block',
  timeRange: 1000
const scrollRange = $scrollingContainer.scrollHeight - $scrollingContainer.clientHeight;

const slowParallax = new WorkletAnimation(
    new KeyframeEffect($parallax_slow, [{'transform': 'translateY(0)'}, {'transform': 'translateY(' + -scrollRange + 'px)'}], scrollRange),
    {rate : 0.4}

const fastParallax = new WorkletAnimation(
    new KeyframeEffect($parallax_fast, [{'transform': 'translateY(0)'}, {'transform': 'translateY(' + -scrollRange + 'px)'}], scrollRange),
    {rate : 0.8}


// Inside AnimationWorkletGlobalScope.
registerAnimator('parallax', class {
  constructor(options) {
    this.rate_ = options.rate;

  animate(currentTime, effect) {
    effect.localTime = currentTime * this.rate_;


WorkletAnimation extends Animation and adds a getter for its timelines. Its constructor takes:

  • animatiorId which should match the id of an animator which is registered in the animation worklet scope.
  • A sequence of effects which are passed into a GroupEffect constructor.
  • A sequence of timelines, the first one of which is considered primary timeline and passed to Animation constructor.
[Constructor (DOMString animatorName,
              optional (AnimationEffectReadOnly or array<AnimationEffectReadOnly>)? effects = null,
              AnimationTimeline? timeline,
              optional WorkletAnimationOptions)]
interface WorkletAnimation : Animation {
        readonly attribute DOMString animatorName;

TODO: At the moment GroupEffect constructor requires a timing but this seems unnecessary for WorkletAnimation where it should be possible to directly control individual child effect local times. We need to bring this up with web-animation spec.

AnimationEffectReadOnly gets a writable localTime attribute which may be used to drive the effect from the worklet global scope.

partial interface AnimationEffectReadOnly {
    // Intended for use inside Animation Worklet scope to drive the effect.
    attribute double localTime;


The draft specification is the most recent version.