Resource guide
2.3.1 Three Flashes or Below Threshold
Content must not flash more than three times per second, unless the flash is small or below brightness thresholds.
By Calling All Minds·Last updated April 2026
Success criterion
Conformance level
Essential baseline — must meet for any compliance.
What it means
Photosensitive epilepsy affects around 3 per 100,000 people and can cause seizures when triggered by flashing or strobing light at certain frequencies. The threshold most associated with seizures is around 3 to 50 flashes per second.
Content must either not flash at all at the dangerous frequency, or the flashing area must be small (below a defined size relative to the viewport) or below brightness and contrast thresholds defined in the WCAG specification.
This is one of the few accessibility failures that poses an immediate physical danger. It must be treated as a priority.
In practice
Do not use strobing effects, rapid cuts, or flashing animations in marketing videos or background content.
If you need to assess whether an existing animation or video passes the threshold, use a tool such as the Photosensitive Epilepsy Analysis Tool (PEAT) or Harding Test.
Animated GIFs, CSS animations, and JavaScript-driven animations can all trigger this criterion.
Common failures
- Strobing background animation on a landing page
- Video with rapid camera flashes or flickering light effects
- Advertisement or banner that flashes at a dangerous frequency
AXS Audit
AXS Audit checks your site against 2.3.1 and flags issues your team can act on straight away. It covers criteria that automated scanners often miss.