Level AWCAG 2.2

2.3.1 Three Flashes or Below Threshold

Web pages do not contain anything that flashes more than three times in any one second period.

Why it matters

Photosensitive seizures
Flashing can trigger seizures in people with photosensitive epilepsy. This is a life-threatening issue.
Migraines
Flashes and rapid flickering can trigger migraines that last for hours or days.
Visual discomfort
Flashing is uncomfortable for many people and severely impairs concentration.
The Pokemon incident
A 1997 TV broadcast sent around 700 people to the hospital. The danger of flashing content is well documented.

Thresholds and safe patterns

Flash thresholds and safe animation

No real flashing is shown here. Instead, this diagram explains the threshold concept and safer alternatives.

Risky patterns
More than 3 flashes per second
Can trigger photosensitive seizures
Red flashes
Especially risky when saturated red blinks
It becomes especially dangerous when the flashing area covers more than 25% of the screen
Safer patterns
Gentle transitions
Avoid abrupt light-dark changes
Gradient animation
Smooth color changes are safer
SLOW
Low-frequency changes (2 or fewer per second)
Safer when kept below the threshold
General flash threshold
>3
flashes per second
Failure
≤3
per second and small area
Conditionally OK
0
No flashes
Safe
Detecting prefers-reduced-motion
Normal mode

Your OS setting: "Reduce motion" is off

@media (prefers-reduced-motion: reduce) {
  .animation {
    animation: none;
    transition: none;
  }
}
Photosensitive seizures are a serious safety issue. Avoid flashing content whenever possible, and if you truly must use it, stay under the threshold and honor prefers-reduced-motion .

Understanding through personas

Yamamoto (16) — Photosensitive epilepsy

A movie site's trailer flashed violently without warning and I had a seizure. Now I ask my family to check pages before I browse. 'Even just a warning would help me feel safe.'

Checkpoints

References