prefers-reduced-motion: which animation to reduce and how

What it is

prefers-reduced-motion is a CSS media feature that, per MDN, detects whether the user has enabled a reduced-motion setting on their device. It has two values: no-preference (evaluates as false) and reduce. MDN's own page carries a warning that its scaling example may be a problem for readers with vestibular motion disorders, which is the group the setting exists for: large moving, zooming or parallax effects can cause dizziness or nausea.

Two WCAG criteria frame the obligation. 2.2.2 Pause, Stop, Hide (level A) requires a way to pause, stop or hide any moving, blinking or scrolling content that starts automatically, lasts more than five seconds and appears alongside other content. 2.3.3 Animation from Interactions (level AAA) asks that motion animation triggered by interaction can be disabled unless it is essential; its example is a page-flip transition that is turned off when the media query reports reduce.

Why it matters

The setting is a user's explicit request. Ignoring it makes a site unusable for some people and unpleasant for many more who enabled it for battery, distraction or preference reasons; honouring it costs a few CSS lines.

How to apply

• Prefer opt-in: wrap motion in @media (prefers-reduced-motion: no-preference) { … } so the default is calm and the animation is the addition. Alternatively override under (prefers-reduced-motion: reduce).
• Reduce, do not necessarily remove. Replace translations, scaling and parallax with opacity fades or instant changes; keep short transitions that show cause and effect (a panel opening, a toggle switching) if they involve little movement.
• Set scroll-behavior: auto instead of smooth under reduce, and stop autoplaying background video and carousels.
• Mirror the query in script: window.matchMedia('(prefers-reduced-motion: reduce)') with a change listener, and pass the result to animation libraries and to Element.animate() calls.
• Provide an in-app motion toggle as well; it covers users whose platform has no setting and satisfies 2.2.2's pause/stop/hide requirement for autoplaying content.

Pitfalls

The blanket hack * { animation: none !important; transition: none !important } breaks code that waits for animationend or transitionend; shortening durations to near zero keeps those events firing. Loading spinners are motion too; a reduced variant (pulsing opacity, static text "Loading…") is still needed. Content that flashes remains a separate concern (WCAG 2.3.1) and is not fixed by this query.