WCAG 2.1 · Level A · Operable
WCAG 2.3.1 — Three Flashes or Below Threshold, explained with examples
Content must not flash more than three times in any one-second period, or the flash must stay below the general flash and red flash thresholds. Photosensitive users can suffer seizures from flashing content. This is a safety criterion, not just an accessibility one.
- Number
- 2.3.1
- Level
- A
- Principle
- Operable
- Guideline
- 2.3 Seizures
Why this criterion exists
Photosensitive users can suffer seizures from flashing content. This is a safety criterion, not just an accessibility one.
If you only remember one thing: content must not flash more than three times in any one-second period, or the flash must stay below the general flash and red flash thresholds. Everything else on this page is detail.
Who feels it when this fails
Accessibility criteria sometimes feel abstract until you see who pays the cost when a site ignores them. Three Flashes or Below Threshold affects:
Users with photosensitive epilepsy
How sites typically fail it
These are the patterns we see week after week. None are intentional — they are accidents of how teams build interfaces under deadline. Knowing the failure modes is the fastest path to writing them out of your component library.
Strobe-like loading animations
Decorative video with rapid cuts
How to test for it
Use the Photosensitive Epilepsy Analysis Tool (PEAT) on suspect media.
Automated scanners catch this criterion most of the time, but never all of the time. Manual testing with the keyboard and a screen reader closes the gap.
A code fix you can copy
Slow the animation, respect reduced-motion, and never show large strobing red areas.
The problem
.flicker { animation: flash 0.1s infinite; }The fix
@media (prefers-reduced-motion: reduce) { .flicker { animation: none; } }
.flicker { animation: flash 0.5s infinite alternate; } /* 2/sec, safe */Slow the animation, respect reduced-motion, and never show large strobing red areas.
Frequently asked questions
How common is photosensitive epilepsy?
Approximately 1 in 4,000 people has photosensitive epilepsy — about 65 million people worldwide. Flashing content can also trigger migraines and vestibular disorders in a much larger population. The 2023 WebAIM Million study found 0.1% of pages had detectable flashing content, but design animations and video content are outside automated scan scope.
Is there a tool to test for flashing content?
The Photosensitive Epilepsy Analysis Tool (PEAT) from the Trace Research Center is the reference tool for evaluating video and animation against the Harding Flash and Pattern Analyzer thresholds. For CSS animations, the rule is simple: never flash faster than 3 times per second over a large screen area, particularly in red.
Other Operable criteria
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