CSS animations off the UI thread

March 12th, 2013. Tagged: CSS, performance

This excellent Google I/O talk mentions that Chrome for Android moves the CSS animations off of the UI thread, which is, of course, a great idea. Playing around with it, here's what I found:

  • Browser support: Desktop Safari, iOS Safari, Android Chrome.
  • You need to use CSS transforms. Animating regular properties doesn't work.

Update: (see comments) confirmed support in IE10. Reported support in Firefox OS too, but I cannot personally confirm

More details and test page below.

Single UI thread

As you probably know the browser is single threaded. Do something heavy in ECMAScript land and everything freezes.

The big idea

CSS animations should be excluded from the "everything" that freezes.

Test page

Here's a test page with some animations. Click the kill button and see what happens.


The red box that spins is animated like:

.spin {
  animation: 3s rotate linear infinite;
@keyframes rotate {
  from {transform: rotate(0deg);}
  to {transform: rotate(360deg);}

The green one is also animated with a transform:

.walkabout-new-school {
  animation: 3s slide-transform linear infinite;
@keyframes slide-transform {
  from {transform: translatex(0);}
  50% {transform: translatex(300px);}
  to {transform: translatex(0);}

The blue one is animated using the margin-left property, not a transform:

.walkabout-old-school {
  animation: 3s slide-margin linear infinite;
@keyframes slide-margin {
  from {margin-left: 0;}
  50% {margin-left: 100%;}
  to {margin-left: 0;}

Kill switch

The kill button just pegs the CPU in a infinite loop for 2 seconds:

function kill() {
  var start = +new Date;
  while (+new Date - start < 2000){}


In non-supporting browsers, which is most of them, the kill switch kills all the animations. Business as usual.

In the supporting browsers (All Safaris and Andriod Chrome) the kill only affects the blue button, the one that animates a CSS property, as opposed to using a CSS transform. But the animations that use a transform keep on going!

Take aways

  1. Rejoice! The future is here! Drink and dance uncontrollably around the campfire!
  2. After you sober up, make sure your CSS animations use transform: where possible
  3. Keep migrating them JS animations to CSS
  4. Bug your browser vendor to support this

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30 Responses

  1. Just to let you know that it works in Firefox OS like it works in Chrome. I think there is some support in Firefox but that isn’t enabled yet. I hope it will soon ;)

  2. FYI, this is a work in progress for Firefox. It is already enabled in FirefoxOS but not yet on other platforms where there’s still some work to be done. You can read http://featherweightmusings.blogspot.fr/2013/02/throttling-off-main-thread-animations.html for more details.

  3. Stoyan, according to the specs, you could simplify your CSS3 keyframes this way:

    @keyframes rotate {
    to {transform: rotate(360deg);}

    @keyframes slide-transform {
    50% {transform: translatex(300px);}

    @keyframes slide-margin {
    50% {margin-left: 100%;}

    I know teaching keyframes is not the purpose of this article, but using the above is much simpler and easier to maintain.

  4. Oddly the blue one still freezes in Chrome mobile.

  5. Hmm, in the current chrome (v25) just the blue one stops. Seems like this technique is very experimental…

  6. This is the best test of “transform” vs “left/right/margin”. Thanks!

  7. Could you somehow still control/target the individual “frames” from JavaScript without blocking the animation (still having it off UI thread)?

    A usage example for this would be to have a complete page flip animation but (with certain scenarios) still be able to control it with mouse/touch. Lets say the complete animation is 20 frames long – so when “following” the mouse/finger and you want it to stop in the middle – you’d for example force it to go to frame 10.

    I realise those are not “real” frames which makes interacting with it all the more difficult + yeah you could just directly manipulate position/transform but the question is if that would break the already “rendered” animation off the UI thread.

  8. @Stoyan, thank you for writing this!

    @All, the Web Animations spec aims to cover your use case: https://dvcs.w3.org/hg/FXTF/raw-file/tip/web-anim/index.html Right now, there’s limited control over CSS Animations from JS and you will likely not get smooth results.

  9. IE10 does the same on both Windows 8 Desktop and Windows RT on Surface (have not tested IE10 on Win7 no on Windows Phone 8).
    Only the blue one stops on “kill”. So seems IE10 already puts animation off the UI thread :)
    I do like IE10 more and more…

  10. Seems like it’s not supported for desktop Chrome. All 3 boxes stop on Chrome 25.0.1364.172 and 27.0.1438.4 canary and prevent the page from scrolling down.

  11. It works as expected on on the default android browser as well (Samsung galaxy S3 – Android 4.1.2)

  12. Works on opera (beta – 14.0.1025.52315) on Android as well.

  13. Yeah, seems fine on Android but Chrome is throwing a googly. So much for its rapid development cycle! Oh well, it will catch on, but the wait for agreed standards and browser adoption always feels like an Aesop’s fable in slow motion.

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  16. CSS animation is a good step forward – we will soon forgot that number of JS code and images required for basic animation on hover etc.

  17. I slightly modified your test and added an animation using the “steps” API. I don’t know why but for some reason the animation isn’t rendered off the UI thread in canary / safari. See: https://gist.github.com/basecode/5298961/raw/7a7ba729b0f7ffa53e92a5f06ba6451ec0b3ef72/thread.html

  18. [...] appearing in Desktop Safari, iOS Safari, and Android Chrome, CSS transforms is no longer part of the UI thread, meaning such animations will not be affected by an overloaded CPU! Check-out the test page, I can [...]

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  21. [...] This is an interesting page. It talks about using pure CSS3 animations. They still freeze in Firefox though – maybe soon FF will fix this. Looks like I should be considering CSS for animation rather that using a GIF. [...]

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  26. […] This is an interesting page. It talks about using pure CSS3 animations. They still freeze in Firefox though – maybe soon FF will fix this. Looks like I should be considering CSS for animation rather that using a GIF. […]

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