2.3.1 Three Flashes or Below Threshold

What does it mean?

Flashing content can trigger seizures in people with photosensitive epilepsy, even if they have never had a seizure before. This criterion sets a hard limit: no part of the page may flash more than three times within any one-second period.

A faster flash is only allowed if it stays below the general flash and red flash thresholds defined by WCAG, which depend on the size of the flashing area and the change in brightness or saturated red. In practice, the simplest and safest rule is: avoid flashing content altogether. This criterion applies even to content that users can pause, because a seizure can happen before they reach the control.

Who does it help?

• People with photosensitive epilepsy
• People with other seizure disorders or migraine triggered by flashing light

Common failures

• A video or animated banner with strobe effects or rapid cuts between light and dark
• A CSS animation that blinks a large element several times per second to draw attention
• An animated GIF with fast alternating bright frames

How we test it

• people only Manual only: nothing useful can be measured. The reviewer tests it directly.

Test question: Nothing flashes more than three times per second.

A person decides every criterion. No criterion ever passes automatically.

How to fix it

.alert { animation: blink 0.2s infinite; }
@keyframes blink { 50% { opacity: 0; } }

.alert { animation: pulse 2s ease-in-out 3; }
@keyframes pulse { 50% { box-shadow: 0 0 0 0.5rem rgb(220 38 38 / 0.3); } }

@media (prefers-reduced-motion: reduce) {
  .alert { animation: none; }
}

The rapid blink becomes a slow, soft pulse that runs a few times only and is switched off for users who prefer reduced motion.

Practical tips

• Check videos before publishing; free photosensitivity analysis tools exist for this purpose.
• Small flashing areas are less risky than large ones, but avoiding flashing is always the safer choice.

Official source

Understanding WCAG 2.2 at the W3C: 2.3.1 Three Flashes or Below Threshold