Seizure disorders

Why it matters

Flashing content on websites and in video can trigger seizures in people with photosensitive epilepsy. Unlike most accessibility issues, this one carries a direct safety risk. The BoK’s coverage of seizure disorders focuses on the design rules that prevent triggering photosensitive responses. (BoK p. 27)

Epilepsy

Epilepsy is a neurological disorder in which the brain produces recurrent, unprovoked seizures. Approximately 50 million people worldwide have epilepsy, making it one of the most common neurological conditions. Seizures can range from brief lapses in awareness to full convulsions. (BoK p. 27)

Photosensitive epilepsy

Photosensitive epilepsy is the subset in which seizures can be triggered by specific visual stimuli — most commonly flickering lights, rapidly flashing images, or high-contrast alternating patterns. Photosensitive epilepsy affects approximately 3–5% of people with epilepsy. (BoK p. 27)

The high-risk flicker range

The BoK identifies a flicker rate range of approximately 16–25 Hz as carrying the highest risk of triggering a photosensitive seizure. This is the “danger zone” for flash frequency. Content flickering within this range poses the greatest hazard. (BoK p. 27)

The WCAG flash threshold

WCAG 2.1 Success Criterion 2.3.1 (Three Flashes or Below Threshold) states that web pages must not contain anything that flashes more than three times in any one second period, unless the flash is below defined thresholds for area and contrast. (BoK p. 28)

The criterion applies to:

• Actual flashes of light (full screen strobing).
• Rapidly alternating high-contrast images or animations.
• Video content with strobing effects.

Design measures

• Do not include rapidly flashing content that exceeds the 3 Hz threshold.
• Avoid high-contrast patterns in combination with flash effects.
• Provide a warning before content known to contain flashes (e.g., before a video with strobing effects), and allow users to opt out or pause.
• Test animations and video with the Photosensitive Epilepsy Analysis Tool (PEAT) or equivalent.

AT solutions

The primary solution is design-level — preventing triggering content from appearing at all. AT in this domain is limited:

• Some users manage epilepsy with medication that reduces photosensitivity.
• Screen-level filters or tinted glasses can reduce sensitivity, but they are not substitutes for compliant design.

Real-world example

A news website includes a headline animation that flashes a high-contrast banner 8 times per second. A visitor with photosensitive epilepsy opens the page in a crowded café and has a seizure. The animation is unnecessary for the editorial content and violates WCAG 2.3.1. Removing it or reducing the flash rate to under 3 per second eliminates the risk with no loss of editorial value.