WCAG 2.3.1 Three Flashes or Below Threshold

Success criterion 2.3.1 protects people with photosensitive seizure disorders. Content that flashes too often over a large enough area can trigger a seizure, and that harm happens instantly. It is level A and part of EN 301 549 (clause 9.2.3.1).

What the criterion requires

Web pages must not contain anything that flashes more than three times in any one-second period, unless the flash is below the general flash and red flash thresholds.

Content is below the threshold if either:

• there are no more than three general flashes and/or three red flashes in any one second; or
• the combined area of flashes happening at the same time is no more than 0.006 steradians within any 10 degree visual field (25 % of that field) at typical viewing distance.

Definitions in WCAG:

• a general flash is a pair of opposing changes in relative luminance of 10 % or more of the maximum, where the darker image is below 0.80 relative luminance;
• a red flash is a pair of opposing transitions involving a saturated red.

For ordinary screens WCAG suggests a 341 × 256 pixel rectangle anywhere on the screen, viewed at 1024 × 768, as the size of that area.

Typical failures on an online shop

• a "Black Friday" or "flash sale" banner that strobes between bright colours;
• product videos or brand clips with camera flashes, lightning or fast cuts between dark and light scenes;
• animated GIF badges ("NEW!", "SALE!") that blink rapidly between red and white;
• a full-screen intro or loading animation with flicker effects.

How to check it

Automated: there is no axe-core rule for this criterion. Measuring flashes needs frame-by-frame video analysis, which a page scan does not do, so it needs a manual check.

By hand: list every video, GIF, animated banner and CSS animation. Watch each one and count fast changes between light and dark or into saturated red. If anything changes more than three times in a second, measure it with a dedicated analysis tool for photosensitive content, such as the free Photosensitive Epilepsy Analysis Tool (PEAT), or remove it.

How to fix it

• Do not use strobing or flickering effects at all; there is rarely a business reason for them.
• Slow animations down so that light/dark changes happen no more than three times per second.
• Keep any necessary flashing small and avoid saturated red.
• Check videos from agencies and influencers before putting them on product pages.
• Respect prefers-reduced-motion for animations in general (a good practice, not a substitute for the flash limits).

Related criteria

• 2.2.2 Pause, Stop, Hide — moving and blinking content must be controllable.
• 1.4.3 Contrast (Minimum) — how relative luminance is used for text contrast.
• WCAG 2.2 checklist — all A and AA criteria in one list.

Frequently asked questions

How many flashes per second does WCAG allow?

No more than three flashes in any one-second period, unless the flashing is below the general flash and red flash thresholds, for example because the flashing area is small.

What is the flash area threshold?

Concurrent flashes must together cover no more than 25 % of any 10 degree visual field at typical viewing distance. WCAG gives a practical guide: a 341 × 256 pixel rectangle on a screen viewed at 1024 × 768.

Can a pause button fix flashing content?

No. A seizure can happen before the user reaches any control, so the flashing itself has to stay within the limits. This is also why 2.3.1 applies to all content on the page.