Timing control for script-based animations

W3C Working Draft 2 June 2011

This Version:
Latest Version:
James Robinson, Google, Inc <jamesr@chromium.org>
Cameron McCormack, Mozilla Corporation <cam@mcc.id.au>


This document defines an API web page authors can use to write script-based animations where the user agent is in control of limiting the update rate of the animation. The user agent is in a better position to determine the ideal animation rate based on whether the page is currently in a foreground or background tab, what the current load on the CPU is, and so on. Using this API should therefore result in more appropriate utilization of the CPU by the browser.

Status of this Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document is the 2 June 2011 First Public Working Draft of the Timing control for script-based animations specification. Please send comments about this document to public-web-perf@w3.org (archived) with [RequestAnimationFrame] at the start of the subject line.

This document is produced by the Web Performance Working Group. The Web Performance Working Group is part of the Rich Web Clients Activity in the W3C Interaction Domain. Changes made to this document can be found in the W3C public Mercurial server.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

1. Introduction

This section is informative.

Animations in web browsers come in two forms: native, declarative ones, such as the <animate> element in SVG, and those that are implemented in script. These script-based animations are most often performed by scheduling a callback using setTimeout or setInterval and making changes to the DOM to effect the animation in that callback.

A disadvantage of this approach is that the author of the animation script has no idea what the ideal frequency for updating their animation is. Instead, the easiest way forward for the author is to simply call setTimeout with a very small value, which in practice will be clamped to some minimum time like 10ms anyway. It likely won’t be the case that 100 updates per second are required for the animation, especially if the page is in a background tab or the browser window is minimized.

The API described in this document allows script authors to request the user agent schedule an animation frame update. The user agent is in a better position to determine how many frames per second to allocate to all of the animations running in the entire browser. If there are many animations active, the user agent can select a frame rate such that all of the animations will run as smoothly as possible. If the page is not currently visible, animations on that page can be throttled heavily so that they do not update often and thus consume little CPU power.


Here is an example of using the API to write a script-based animation.

<!DOCTYPE html>
<title>Script-based animation using requestAnimationFrame</title>
div { position: absolute; left: 10px; padding: 50px;
  background: crimson; color: white }
var requestId = 0;

function animate(time) {
  document.getElementById("animated").style.left =
    (time - animationStartTime) % 2000 / 4 + "px";
  requestId = window.requestAnimationFrame(animate);
function start() {
  animationStartTime = Date.now();
  requestId = window.requestAnimationFrame(animate);
function stop() {
  if (requestId)
  requestId = 0;
<button onclick="start()">Click me to start!</button>
<button onclick="stop()">Click me to stop!</button>
<div id="animated">Hello there.</div>

2. Conformance

Everything in this specification is normative except for diagrams, examples, notes and sections marked as being informative.

The keywords “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY” and “OPTIONAL” in this document are to be interpreted as described in Key words for use in RFCs to Indicate Requirement Levels. [RFC2119]

The IDL fragment in section 4 of this specification MUST be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WEBIDL]

This specification defines a single conformance class:

conforming user agent
A user agent is considered to be a conforming user agent if it satisfies all of the MUST-, REQUIRED- and SHALL-level criteria in this specification. A conforming user agent MUST also be a conforming implementation of the IDL fragment in section 4 of this specification, as described in the Web IDL specification. [WEBIDL]

This specification references interfaces and types from a number of other specifications:

3. Definitions

Associated with every Document is an animation frame request callback list, which is a list of <handle, callback, cancelled> triples. handle is an integer that uniquely identifies the entry in the list, callback is a FrameRequestCallback object, and cancelled is a Boolean flag. Initially, the animation frame request callback list for a Document is empty.

A Document is said to have active animations whenever it has a non-empty animation frame request callback list.

4. The WindowAnimationTiming interface

The WindowAnimationTiming interface is used to expose the requestAnimationFrame operation on the Window object. In the definition of requestAnimationFrame below, references to the Document object are to be taken to be references to the Window object’s active document. [HTML5]

[Supplemental, NoInterfaceObject]
interface WindowAnimationTiming {
  long requestAnimationFrame(in FrameRequestCallback callback);
  void cancelRequestAnimationFrame(in long handle);

Window implements WindowAnimationTiming;

[Callback, NoInterfaceObject]
interface FrameRequestCallback {
  void sample(in DOMTimeStamp time);

The requestAnimationFrame method is used to signal to the user agent that a script-based animation needs to be resampled. When requestAnimationFrame(callback) is called, the user agent MUST schedule a script-based animation resampling by appending to the end of the animation frame request callback list an entry whose handle is a user-agent-defined integer greater than zero that uniquely identifies the entry in the list, whose callback is callback and whose cancelled flag is false.


requestAnimationFrame only schedules a single update to the script-based animation. If subsequent animation frames are needed, then requestAnimationFrame will need to be called again from within the callback.

Also note that multiple calls to requestAnimationFrame with the same callback (before callbacks are invoked and the list is cleared) will result in multiple entries being in the list with that same callback, and thus will result in that callback being invoked more than once for the animation frame.

Editorial note

ISSUE-4 Do we want to allow an Element to be passed to requestAnimationFrame, so that animations affecting the given element are throttled or paused when scrolled out of view?

The cancelRequestAnimationFrame method is used to cancel a previously made request to schedule an animation frame update. When cancelRequestAnimationFrame(handle) is called, the user agent MUST set the cancelled flag to true for the entry in the animation frame request callback list whose handle is handle. If there is no entry in the list with the given handle, then this function does nothing.


cancelRequestAnimationFrame might be called for an entry in the Document’s animation frame request callback list or in the sample all animations operation’s temporary list. In either case the entry’s cancelled flag is set to true so that the callback does not run.

While a Document has active animations, the user agent MUST continually sample all animations. To sample all animations, the following steps are performed:

Editorial note

ISSUE-1 We should define the processing model in terms of an HTML5 task queue, so that animation frame callback scheduling participates in the browser event loop in a well defined manner, rather than just saying that animations are sampled continually.

  1. Let t be the result of getting the next sample time of the Document.
    Editorial note

    ISSUE-2 Getting the next sample time is currently undefined. This time should be the same as the time that contemporaneous native animations have been sampled at.

  2. Let time be t expressed as the number of milliseconds since 1970-01-01T00:00:00Z.
    Editorial note

    ISSUE-3 Having animation frame times run off a monotonic clock that increases at a constant rate would be better for authors than using the wallclock time as reported by the system, which might jump backwards or move forwards at varying rates due to clock slew. Doing this argues for the reinclusion of the Window.animationStartTime attribute that was present in an earlier draft, so that scripts can avoid using Date.now() to record the animation start time, which might bear little relation to the monotonic clock values anyway.

  3. Let list be a copy of the Document’s animation frame request callback list. list will be empty if no script-based animations have been scheduled.
  4. Clear the Document’s animation frame request callback list.
  5. Perform the steps defined in the invoke callbacks algorithm with parameters list and time

The expectation is that the user agent will never run the invoke callbacks algorithm more than once per display refresh, and that when there is no need to throttle down the animation sample rate, it will attempt to run the invoke callbacks algorithm at exactly once per display refresh.

Editorial note

ISSUE-5 Do we need to set expectations on exactly how rate throttling is done, and if callback invocation can be suspended indefinitely?

The invoke callbacks algorithm:

  1. For each entry callback in list, in order:
    1. If the cancelled flag on callback is not true:
      1. Call the sample operation of callback with time as the argument.
      2. If calling the operation resulted in an exception being thrown, then catch that exception and ignore it.

5. Acknowledgements

This section is informative.

The editors would like to thank the following people for contributing to this specification: Boris Zbarsky, Jonas Sicking, Robert O’Callahan.

A. References

A.1. Normative references

Document Object Model (DOM) Level 3 Core Specification. A. Le Hors, et al., Editors. World Wide Web Consortium, April 2004. Available at http://www.w3.org/TR/2004/REC-DOM-Level-3-Core-20040407/.
HTML5. I. Hickson, Editor. World Wide Web Consortium, June 2010. Available at http://www.w3.org/TR/2010/WD-html5-20100624/.
Key words for use in RFCs to Indicate Requirement Levels, S. Bradner. IETF, March 1997. Available at http://www.ietf.org/rfc/rfc2119.
Web IDL. C. McCormack, Editor. World Wide Web Consortium, October 2010. Available at http://www.w3.org/TR/2010/WD-WebIDL-20101021/.

A.2. Informative references

B. Changes

The following is a list of substantial changes to the document on each publication.

2 June 2011 – FPWD
  • Initial publication.