Success Criteria

WCAG 2.3.1 Three Flashes or Below Threshold

Success Criterion 2.3.1 requires that content doesn't contain anything that flashes more than three times in any one-second period, unless the flash is below a defined general and red-flash threshold. It's a Level A criterion under Guideline 2.3 (Seizures and Physical Reactions) — one of the few WCAG criteria with a genuine, serious physical-safety dimension rather than a usability one.

Why this criterion exists

Rapid, high-contrast flashing can trigger seizures in people with photosensitive epilepsy — this isn't a comfort or convenience issue like most WCAG criteria, it's a direct physical safety hazard. This is reflected in how strictly the criterion is defined: it doesn't rely on subjective judgment, it specifies an exact, testable flash-rate and contrast threshold.

The technical threshold, briefly

Content fails this criterion if it flashes more than 3 times per second AND the flash is large enough and contrasty enough to exceed defined general and red-flash thresholds (the exact thresholds involve specific luminance and screen-area calculations defined in the WCAG technique documentation). In practice, testing this precisely typically requires specialized tools built for exactly this purpose — the Photosensitive Epilepsy Analysis Tool (PEAT) being the standard one — rather than a visual judgment call.

Where this shows up

• Auto-playing video content with rapid strobe-like effects
• Animated GIFs or CSS animations with fast, high-contrast flashing
• Poorly-optimized ads (this is one of the reasons third-party ad content is a recurring source of accessibility risk beyond just the usual missing-alt-text/contrast issues)

The practical stance

Because the safety stakes are real and the threshold calculation is specialized, the safest practical approach for any content with genuine flashing or strobing effects is to test it with a dedicated tool like PEAT before publishing, rather than eyeballing it — and where there's real doubt, redesigning the effect to flash less than 3 times per second removes the risk entirely regardless of the exact contrast/area math.

Common questions

What is WCAG 2.3.1?

A Level A criterion: content must not flash more than three times in any one second, or must stay under the defined general and red flash thresholds.

Why does 2.3.1 matter?

Rapid flashing, especially in red, can trigger seizures in people with photosensitive epilepsy.

How do I comply?

Avoid content that flashes more than three times per second; if flashing is needed, keep the flashing area small and below the luminance and red thresholds.