WCAG 2.3.1 — Three Flashes or Below Threshold

What this requires

Content must not contain anything that flashes more than three times in any one-second period, unless the flashing area is small and the flash stays below the general and red flash thresholds defined in WCAG. The conservative reading: don't flash more than three times per second, full stop, unless you've measured the flash area and luminance against the technical thresholds.

How AI coding tools fail this

When asked for "an attention-grabbing loading state" or "a high-energy hero animation", AI tools sometimes generate strobing colour transitions, rapid-fire opacity or background keyframes, or flashing notification badges. The visual reads as energetic; the medical risk for photosensitive users is real.

The second pattern: GIFs and short videos with rapid flash sequences embedded with no warning and no autoplay control. Even when the flash isn't in the page CSS, embedding a 15Hz strobe loop is the same failure with a different delivery method.

The third: chart libraries with default animations that flash data points in rapid sequence as the user interacts with filters. The flashing is incidental but real.

Failing examples

A strobing CSS keyframe:

@keyframes alert-flash {
  0%, 100% { background: red; }
  50% { background: yellow; }
}

.alert-flash {
  animation: alert-flash 0.1s infinite;
}

Autoplaying a GIF with rapid flashing:

<img src="/strobe-loader.gif" alt="" />

Passing examples

Use a slow pulse — well under three flashes per second:

@keyframes alert-pulse {
  0%, 100% { box-shadow: 0 0 0 0 rgba(220, 38, 38, 0.5); }
  50% { box-shadow: 0 0 0 8px rgba(220, 38, 38, 0); }
}

.alert-pulse {
  animation: alert-pulse 1.6s ease-out infinite;
}

Respect prefers-reduced-motion:

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

Edge cases

The general flash threshold is roughly: a flash with a relative luminance change of less than 0.1, or covering less than 25% of the 10° visual field at typical viewing distance, is below threshold. Most teams should treat anything over 3 Hz as a fail rather than trying to measure the threshold.
Red flashing is more dangerous than other colours and has a stricter threshold.
prefers-reduced-motion doesn't satisfy this criterion alone — the criterion is about absolute danger, not user preference — but it's a good companion.
Test with PEAT (the Photosensitive Epilepsy Analysis Tool from the Trace Center) when in doubt.
2.3.1 is A; 2.3.2 is AAA. The difference: 2.3.2 removes the small-area exception entirely.

How Jeikin handles this

This criterion is genuinely hard to verify statically — frame-rate analysis requires runtime measurement. Jeikin's guided review uses PEAT on flagged components and records the result. The scanner flags CSS animations with durations under 333ms (the 3 Hz floor) as leading indicators.