Understanding Success Criterion 2.3.2: Three Flashes

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

Intent of Success Criterion 2.3.2: Three Flashes

The purpose of this Success Criterion is to further reduce the chance of seizures. Seizures cannot be completely eliminated since some people are so sensitive. However, by eliminating all 3-per-second flashing over any area of the screen, the chances of a person having a seizure are further reduced than when just meeting the measures ordinarily used today in standards internationally, as we do at Level A.

Whereas Success Criterion 2.3.1 allows flashing if it is dim enough or has a small enough area, Success Criterion 2.3.2 does not allow flashing greater than 3 per second, regardless of brightness or size. As a result, even a single flashing pixel would violate this criterion. The intent is to guard against flashing larger than a single pixel, but since an unknown amount of magnification or high contrast setting may be applied, the prohibition is against any flashing.

In some cases, what we refer to as "blinking" and what we refer to as "flashing" may overlap slightly. We are using different terms for the two because "blinking" causes a distraction problem which you can allow for a short time as long as it stops (or can be stopped) whereas "flashing" is a seizure trigger and cannot be allowed or it will cause a seizure. The seizure would occur faster than most users could turn it off. "Blink" therefore refers to slow repeating changes that would distract. "Flash" refers to changes that could cause a seizure if they were bright enough or persisted long enough. Blinking usually doesn't occur at speeds of 3 per second or more so blink and flash do not overlap. However, blinking can occur faster than 3 per second so there could be an overlap. See for more information on blink.

Benefits of Success Criterion 2.3.2: Three Flashes

  • Individuals who have seizures when viewing flashing material will be able to view all of the material on a site without having a seizure and without having to miss the full experience of the content by being limited to text alternatives. This includes people with photosensitive epilepsy as well as other photosensitive seizure disorders.

Examples of Success Criterion 2.3.2: Three Flashes

  • A movie with a scene involving very bright lightning flashes is edited so that the lightning only flashes three times in any one second period.

Resources Success Criterion 2.3.2: Three Flashes

Techniques for Success Criterion 2.3.2: Three Flashes

Sufficient Techniques

  1. Ensuring that no component of the content flashes more than three times in any one second period

Advisory Techniques

  • Reducing contrast for any flashing content (future link)
  • Avoiding fully saturated reds for any flashing content (future link)
  • Reducing the number of flashes even if they don't violate thresholds (future link)
  • Slowing down live material to avoid rapid flashes (as in flashbulbs) (future link)
  • Freezing the image momentarily if 3 flashes within one second are detected (future link)
  • Dropping the contrast ratio if 3 flashes within one second are detected (future link)

Failures