4 of 86 rules
Seizures and vestibular disorders
This is the smallest set on the site and the only one where getting it wrong makes somebody physically ill. Flashing can trigger a seizure. Large moving imagery can bring on nausea, migraine, and dizziness bad enough that the guidance behind these rules talks about needing bed rest afterwards. None of it is a matter of taste, and none of it is answered by telling people to look away.
What tends to break
The flash rule counts, and it counts inside a window rather than as a rate. Three flashes in any one second are allowed and a fourth is not, unless the flash stays below the general and red flash thresholds. A small enough flashing area clears those thresholds on its own, and small here means a quarter of a ten degree field of vision, which W3C works out at roughly 21,824 square pixels on a standard screen at a normal viewing distance. Motion is the other half, and it splits by who started it. Motion the page starts by itself, autoplaying video and a carousel that advances on its own, is Level A, which puts it inside the scope of any audit anybody runs on you. Motion your visitor started by scrolling or hovering is Level AAA. Parallax is the named example there, and scrolling itself is fine, because the problem was never movement but movement nobody asked for. Honour the reduce-motion setting and most of this goes away. Be careful with the mechanism, though. The eye-and-inner-ear explanation everyone repeats is the leading theory rather than settled fact, and the harm is real whatever turns out to cause it.
Level A, 2 rules
- 2.2.2 Pause, Stop, Hide
This affects people with ADHD and other cognitive disabilities, people with vestibular disorders who get dizzy or nauseous from on-screen motion, screen-magnifier users trying to track a target that keeps moving, screen-reader users whose linear read is interrupted by content changing underneath them, and anyone who reads stationary text slowly.
- 2.3.1 Three Flashes or Below Threshold
This affects people with photosensitive epilepsy, for whom a failure is a medical risk rather than an inconvenience, and it affects visitors who get migraines, dizziness or nausea from flashing content without ever having a seizure.
Level AAA, 2 rules
No law names AAA as a blanket duty, but these are where the remaining barriers for people for whom motion or flashing causes harm usually sit.
- 2.3.2 Three Flashes
People with photosensitive epilepsy and other photosensitive seizure disorders are who this protects.
- 2.3.3 Animation from Interactions
People with vestibular disorders feel this most, along with people prone to migraine and people with general motion sensitivity.
How this list was made
Each rule appears here because its own note about who it affects names people for whom motion or flashing causes harm. That keeps this page and the rule pages saying the same thing, and it makes the list narrower than every rule that could matter, because a rule helping these readers without saying so does not turn up. Start with this, then read the full Level AA set, which is what the laws actually name.
Other groups
- Blind and screen reader users40 rules
- Low vision35 rules
- Deaf and hard of hearing9 rules
- Motor and keyboard-only33 rules
- Voice control13 rules
- Cognitive and learning50 rules