Monday, 15 May, 2017 UTC


WebKit now supports the prefers-reduced-motion media feature, part of CSS Media Queries Level 5, User Preferences. The feature can be used in a CSS @media block or through the window.matchMedia() interface in JavaScript. Web designers and developers can use this feature to serve alternate animations that avoid motion sickness triggers experienced by some site visitors.
To explain who this media feature is for, and how it’s intended to work, we’ll cover some background. Skip directly to the code samples or prefers-reduced-motion demo if you wish.

Motion as a Usability Tool

CSS transforms and animations were proposed by WebKit engineers nearly a decade ago as an abstraction of Core Animation concepts wrapped in a familiar CSS syntax. The standardization of CSS Transforms/CSS Animations and adoption by other browsers helped pave the way for web developers of all skill levels. Richly creative animations were finally within reach, without incurring the security risk and battery cost associated with plug-ins.
The perceptual utility of appropriate, functional motion can increase the understandability and —yes— accessibility of a user interface. There are numerous articles on the benefits of animation to increase user engagement:
  • Issara Willenskomer: Creating Usability with Motion: The UX in Motion Manifesto
  • Aurora Harley: Animation for Attention and Comprehension
  • Val Head: UI Animation and UX: A Not-So-Secret Friendship
In 2013, Apple released iOS 7, which included heavy use of animated parallax, dimensionality, motion offsets, and zoom effects. Animation was used as tool to minimize visual user interface elements while reinforcing a user’s understanding of their immediate and responsive interactions with the device. New capabilities in web and native platforms like iOS acted as a catalyst, leading the larger design community to a greater awareness of the benefits of user interface animation.
Since 2013, use of animation in web and native apps has increased by an order of magnitude.

Motion is Wonderful, Except When it’s Not

Included in the iOS accessibility settings is a switch titled “Reduce Motion.” It was added in iOS 7 to allow users the ability to disable parallax and app launching animations. In 2014, iOS included public API for native app developers to detect Reduce Motion (iOS, tvOS) and be notified when the iOS setting changed. In 2016, macOS added a similar user feature and API so developers could both detect Reduce Motion (macOS) and be notified when the macOS pref changed. The prefers-reduced-motion media feature was first proposed to the CSS Working Group in 2014, alongside the release of the iOS API.
Wait a minute! If we’ve established that animation can be a useful tool for increasing usability and attention, why should it ever be disabled or reduced?
The simplest answer is, “We’re not all the same.” Preference is subjective, and many power users like to reduce UI overhead even further once they’ve learned how the interface works.
The more important, objective answer is, “It’s a medical necessity for some.” In particular, this change is required for a portion of the population with conditions commonly referred to as vestibular disorders.

Vestibular Spectrum Disorder

Vestibular disorders are caused by problems affecting the inner ear and parts of the brain that control balance and spatial orientation. Symptoms can include loss of balance, nausea, and other physical discomfort. Vestibular disorders are more common than you might guess: affecting as many as 69 million people in the United States alone.
Most people experience motion sickness at some point in their lives, usually while traveling in a vehicle. Consider the last time you were car-sick, sea-sick, or air-sick. Nausea can be a symptom of situations where balance input from your inner ear seems to conflict with the visual orientation from your eyes. If your senses are sending conflicting signals to your brain, it doesn’t know which one to trust. Conflicting sensory input can also be caused by neurotoxins in spoiled food, hallucinogens, or other ingested poisons, so a common hypothesis is that these conflicting sensory inputs due to motion or vestibular responses lead your brain to infer its being poisoned, and seek to expel the poison through vomiting.
Whatever the underlying cause, people with vestibular disorders have an acute sensitivity to certain motion triggers. In extreme cases, the symptoms can be debilitating.
Vestibular Triggers
The following sections include examples of common vestibular motion triggers, and variants. If your site or web application includes similar animations, consider disabling or using variants when the prefers-reduced-motion media feature matches.

Trigger: Scaling and Zooming

Visual scaling or zooming animations give the illusion that the viewer is moving forward or backward in physical space. Some animated blurring effects give a similar illusion.
Note: It’s okay to keep many real-time, user-controlled direct manipulation effects such as pinch-to-zoom. As long as the interaction is predictable and understandable, a user can choose to manipulate the interface in a style or speed that works for their needs.

Example 1: Mouse-Triggered Scaling

How to Shoot on iPhone incorporates a number of video and motion effects, including a slowly scaling poster when the user’s mouse hovers over the video playback buttons.
The team implemented prefers-reduced-motion to disable the scaling effect and background video motion.

Example 2: 3D Zoom + Blur

The macOS web site simulates flying away from Lone Pine Peak in the Sierra Nevada mountain range. A three-dimensional dolly zoom and animated blur give the viewer a sense that physical position and focal depth-of-field is changing.
In mobile devices, or in browsers that can’t support the more complicated animation, the effect is reduced to a simpler scroll view. By incorporating similar visual treatment, the simpler variant retains the original design intention while removing the effect. The same variant could be used with prefers-reduced-motion to avoid vestibular triggers.

Trigger: Spinning and Vortex Effects

Effects that use spiraling or spinning movements can cause some people with vestibular disorders to lose their balance or vertical orientation.

Example 3: Spinning Parallax Starfield

Viljami Salminen Design features a spinning, background star field by default.
It has incorporated prefers-reduced-motion to stop the spinning effect for users with vestibular needs. (Note: The following video is entirely motionless.)

Trigger: Multi-Speed or Multi-Directional Movement

Parallax effects are widely known, but other variants of multi-speed or multi-directional movement can also trigger vestibular responses.

Example 4: iOS 10 Site Scrolling

The iOS 10 site features images moving vertically at varying speeds.
A similar variant without the scroll-triggered image offsets could be used with prefers-reduced-motion to avoid vestibular triggers.

Trigger: Dimensionality or Plane Shifting

These animations give the illusion of moving two-dimensional (2D) planes in three-dimensional (3D) space. The technique is sometimes referred to as two-and-a-half-dimensional (2.5D).

Example 5: Plane-Shifted Scrolling

Apple’s Environment site features a animated solar array that tilts as the page scrolls.
The site supports a reduced motion variant where the 2.5D effect remains a still image.

Trigger: Peripheral Motion

Horizontal movement in the peripheral field of vision can cause disorientation or queasiness. Think back to the last time you read a book while in a moving vehicle. The center of your vision was focused on the text, but there was constant movement in the periphery. This type of motion is fine for some, and too much to stomach for others.

Example 6: Subtle, Constant Animation Near a Block of Text

After scrolling to the second section on Apple’s Environment site, a group of 10-12 leaves slowly floats near a paragraph about renewable energy.
In the reduced motion variant, these leaves are stationary to prevent peripheral movement while the viewer focuses on the nearby text content.
Take note that only the animations known to be problematic have be modified or removed from the site. More on that later.
Using Reduce Motion on the Web
Now that we’ve covered the types of animation that can trigger adverse vestibular symptoms, let’s cover how to implement the new media feature into your projects.

CSS @Media Block

An @media block is the easiest way to incorporate motion reductions into your site. Use it to disable or change animation and transition values, or serve a different background-image.
@media (prefers-reduced-motion) {
  /* adjust motion of 'transition' or 'animation' properties */
Review the prefers-reduced-motion demo source for example uses.

MediaQueryList Interface

Animations and DOM changes are sometimes controlled with JavaScript, so you can leverage the prefers-reduced-motion media feature with window.matchMedia and register for an event listener whenever the user setting changes.
var motionQuery = matchMedia('(prefers-reduced-motion)');
function handleReduceMotionChanged() {
  if (motionQuery.matches) {
    /* adjust motion of 'transition' or 'animation' properties */
  } else { 
    /* standard motion */
handleReduceMotionChanged(); // trigger once on load if needed
Review the prefers-reduced-motion demo source for example uses.

Using the Accessibility Inspector

When refining your animations, you could toggle the iOS Setting or macOS Preference before returning to your app to view the result, but this indirect feedback loop is slow and tedious. Fortunately, there’s a better way.
The Xcode Accessibility Inspector makes it easier to debug your animations by quickly changing any visual accessibility setting on the host Mac or a tethered device such as an iPhone.
  1. Attach your iOS device via USB.
  2. Select the iOS device in Accessibility Inspector.
  3. Select the Settings Tab.
Alternate closed-captioned version of the Accessibility Inspector demo below.

Don’t Reduce Too Much

In some cases, usability can suffer when reducing motion. If your site uses a vestibular trigger animation to convey some essential meaning to the user, removing the animation entirely may make the interface confusing or unusable.
Even if your site uses motion in a purely decorative sense, only remove the animations you know to be vestibular triggers. Unless a specific animation is likely to cause a problem, removing it prematurely only succeeds in making your site unnecessarily boring.
Consider each animation in its context. If you determine a specific animation is likely to be a vestibular trigger, consider serving an alternate, simpler animation, or display another visual indicator to convey the intended meaning.
  1. Motion can be a great tool for increasing usability and engagement, but certain visual effects trigger physical discomfort in some viewers.
  2. Avoid vestibular trigger animations where possible, and use alternate animations when a user enables the “Reduce Motion” setting. Try out these settings, and use the new media feature when necessary. Review the prefers-reduced-motion demo source for example uses.
  3. Remember that the Web belongs to the user, not the author. Always adapt your site to fit their needs.

More Information

  • CSS Media Queries Level 5, User Preferences
  • WWDC 2016: Auditing your Apps for Accessibility
  • GDC 2015: Designing to Minimize Simulation Sickness in VR Games
  • CSS Tricks: An Introduction to the Reduced Motion Media Query
  • A List Apart: Designing Safer Web Animation For Motion Sensitivity
  • WCAG 2.0, Guideline 2.3: Avoid Content Known to Cause Seizures