Proposal for a new primitive for creating scroll-linked and other high performance procedural animations on the web.
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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 animate a small subset of accelerated properties inside an isolated execution environment, worklet. We believe this API hits a sweet spot, one that is currently missing in the platform, in balancing among performance, richness, and rationality for addressing our key use cases. In particular by limiting ourselves to a subset of accelerated properties we give up some richness to gain performance while maintaining rationality. Finally, it is possible to fine tune this trade-off in future iteration of this API by exposing additional options and without fundamentally reworking this design.

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:

    • 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.


Below is a set of simple examples to showcase the proposed syntax and API usage. Like other Houdini APIs they all rely on first importing a script into the scope of a worklet:

if (animationWorklet)
  // AnimationWorklet not supported, use legacy animation fallback or polyfill

Example 1. A fade-in animation with spring timing

Register the animator in AnimationWorklet scope:

registerAnimator('spring-fadein', class SpringAnimator {
  static inputProperties =  ['--spring-k'];
  static outputProperties =  ['opacity'];
  static inputTime = true;

  animate(root, children, timeline) {
    children.forEach(elem => {
      // read a custom css property.
      const k = elem.styleMap.get('--spring-k') || 1;
      // compute progress using a fancy spring timing function.
      const effectiveValue = this.springTiming(timeline.currentTime, k);
      // update opacity accordingly.
      elem.styleMap.opacity = effectiveValue;

  springTiming(timestamp, k) {
    // calculate fancy spring timing curve and return a sample.
    return 0.42;


Assign elements to the animator declaratively in CSS:

.myFadein {

<section class='myFadein'></section>
<section class='myFadein' style="--spring-k: 25;"></section></pre>

Example 2. Multiple Parallax animations

Register the animator in AnimationWorklet scope:

registerAnimator('parallax', class ParallaxAnimator {
  static inputProperties = ['transform', '--parallax-rate'];
  static outputProperties = ['transform'];
  static rootInputScroll = true;

  animate(root, children) {
    // read scroller's vertical scroll offset.
    const scrollTop =;
    children.forEach(background => {
        // read parallax rate.
        const rate = background.styleMap.get('--parallax-rate');
        // update parallax transform.
        let t = background.styleMap.transform;
        t.m42 =  rate * scrollTop;
        background.styleMap.transform = t;

Assign elements to the animator declaratively in CSS:

:root {
  animator-root: parallax;

.bg {
  animator: parallax;
  position: fixed;
  opacity: 0.5;
  z-index: -1;

<div class='bg' style='--parallax-rate: 0.2'></div>
<div class='bg' style='--parallax-rate: 0.5'></div>

Define Custom CSS properties in document Scope:

  name: '--parallax-rate',
  inherits: false,
  initial: 0.2,
  syntax: '<number>'

Example 3. Position Sticky

Register the animator in AnimationWorklet scope:

// worklet scope

registerAnimator('top-sticky', class TopStickyAnimator {
  static inputProperties = ['--trigger-point'];
  static outputProperties = ['transform'];
  static rootInputScroll = true;

  animate(root, children) {
    // read scroller's vertical scroll offset.
    const scrollTop =;

   children.forEach(sticky => {
        const triggerPoint = child.styleMap.get('--trigger-point');
        // if we have scrolled passed the trigger point the sticky element needs
        // to behave as if fixed. We do this by using a transform to position
        // the element relative to its container.
        const stickyOffset = Math.max(0 , scrollTop - triggerPoint);
        sticky.styleMap.transform = new CSSTransformMatrix({m42: stickyOffset});

Assign elements to the animator in document scope:

.sticky_container {
  animator-root: top-sticky;
  overflow: scroll;

.sticky {
  animator: top-sticky;


<div class="sticky_container">
   <!-- Next element is both a sticky root and a sticky child. :) -->
   <div class="sticky_container sticky">
      <div class="sticky"></div>

onload = (e) => {
  document.querySelectorAll('.sticky').forEach(stickyEl => {

  const scrollerEl = findAncestorScroller(stickyEl);
  // calculate scroll trigger point based on stickyEl BCR and scrollerEl BCR and scrollHeight
  const triggerPoint = calculateLimits(stickyEl, scrollerEl); // e.g., 100 = '--trigger-point: ' + triggerPoint + 'px';


Example 4. Synced Scroller

Register the animator in AnimationWorklet scope:

// worklet scope

registerAnimator('sync-scroller', class SyncScrollerAnimator {
  static get inputProperties = ['--scroller-type'];
  static get inputScroll = true;
  static get outputScroll = true;

  animate(root, children) {
    var input = children.filter(e => { return e.styleMap.get("--scroller-type") == "input"})[0];
    var outputs = children.filter(e => { return e.styleMap.get("--scroller-type") == "output"});

    if (!input)

    outputs.forEach(elem => { =;
      elem.scrollOffsets.left = input.scrollOffsets.left;

Assign elements to the animator in document scope:

  .scroller {
    overflow-y: scroll;

  #main_scroller {
    --animator: sync-scroller;
    --scroller-type: input;

  #alt_scroller {
    --animator: sync-scroller;
    --scroller-type: output;

<div id="main_scroller" class="scroller">
  <div>main content.</div>
<div id="alt_scroller" class="scroller">
  <div>some other content that scroll in sync.</div>

Key Concepts

Animator Root & Child Elements

Each animator instance is associated with a single root element and optionally many child elements. Animator child elements will come from the root element’s DOM sub-tree. These two category of elements have their own defined inputs and outputs.

Two CSS properties (animator and animator-root) may be used to assign an HTML elements to an animator instance either as a root or a child.

The ability to operate (read/write attributes) on many elements in a single callback enables powerful effects that are very hard to achieve if an animator was to operate on a single element.

Root Element

The document’s root element is the default root element for each registered animator. Additional HTML elements may be designated as root element for a given animator using animator-root: animator-identifier CSS syntax. A new animator instance is constructed for each root element and its lifetime is tied to the element’s lifetime. The root element and all the child elements in its DOM sub-tree associated with this animator name get assigned to this animator instance. See below for more details.

Child Elements

Any HTML element may be assigned to an animator instance using animator: animator-identifier CSS syntax. In this case, the element is assigned as a child element to the first ancestor animator of the given name.

Note: The animator receives this in a flat list but we can also consider sending a sparse tree instead.

Assigning an Element to an Animator Instance

As discussed, an animator gets assigned a single root element and optionally many children elements. Assigning an element to an animator instance involves the following:

  1. The element would be treated as if it has an will-change attribute for each of its output attributes.
  2. An AnimationProxy is constructed for the element allowing read and write to declared input and output properties.
    1. For any input property defined for this element, the animation proxy’s style map is populated with its computed value.
    2. Similarly the animation proxy’s scroll offset is populated by the element’s scroll offset.
  3. The constructed proxy is passed to the animator instance in the worklet scope and becomes available to the animate callback in its next call.
  4. Once animate is invoked, the new output values in style map (and similarly output scroll offsets) are used to update the corresponding effective values and visuals. Eventually these values will be synced backed to the main thread.

Explicit Input and Output Attributes

All input and output of the animation are declared explicitly. This allows implementations to do optimizations such as not calling the *animate *callback if no input has changed or if no outputs have any visible effect. The following inputs and outputs may be declared on the animator class using static attributes:


  • Regular and Custom CSS Properties - inputProperties and rootInputProperties (list of identifiers)
  • Scroll Offset - inputScroll and rootInputScroll (boolean)
  • Time - inputTime (boolean)


  • “Fast” CSS Properties - outputProperties and rootOutputProperties (list of identifiers)
  • Scroll Offset - outputProperties and rootOutputProperties (boolean)

Falling Out of Sync

The API is designed to allow animation worklets to run on threads other than the main thread. In particular, it is recommended to run them in a dedicated thread and provide a best-effort attempt to run in sync with frame production. This ensures the animations will not be impacted by jank on main thread. It is still possible for such animation to slip in relation with frame production if animate callback cannot be completed in time. We believe such slippage is going to be rare because:

  • There are no other tasks running on the thread.
  • The exposed features are limited to the fast subset (see below).

Limiting Mutations to Fast Subset

We initially plan to limit the mutable attributes to a "fast" subset which can be mutated on the fast path of almost all modern browsers.

Proposed Mutable Attributes:

  • Scroll Offsets
  • Transform
  • Opacity

This is necessary to ensure animations can run on the the fast path. In future we may consider supporting all animatable properties which means running the worklet on main thread. The animator definition surfaces enough information that makes it possible to decide the target executing thread in advance.

Relationship to Web-Animation/CSS Transition

Effective values from animator gets synced back to the main thread. These values sit at the top of the animation effect stack meaning that they overrides all values from other animations (except CSS transitions). The value is to be treated as a forward-filling animation with a single key-frame i.e., the effective value remains in effect until there is a new value from animator.

CSS Syntax

animator: [ <animator-id> ]#
animator-root: [ <animator-id> ]#

where <animator-id> is a <custom-ident>


partial interface Window {
    [SameObject] readonly attribute Worklet animationWorklet;
callback VoidFunction = void (); // a JS class

interface AnimationWorkletGlobalScope : WorkletGlobalScope {
    void registerAnimator(DOMString name, VoidFunction animatorCtor);
// animationCtor should be a class with the following structure
callback interface AnimatorCtor {
    static boolean inputTime = false;
    static inputProperties = [];
    static outputProperties = [];
    static rootInputProperties = [];
    static rootOutputProperties = [];
    static boolean inputScroll = false;
    static boolean outputScroll = false;
    static boolean rootInputScroll = false;
    static boolean rootOutputScroll = false;
    void animate(AnimationProxy root, sequence<AnimationProxy> children, optional AnimationTimeline timeline);
dictionary ScrollOffests {
    unrestricted double left;
    unrestricted double top;

] interface AnimationProxy {
    attribute ScrollOffests scrollOffsets;
    attribute StylePropertyMap styleMap;