What the standard says
Web pages do not contain anything that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds.
Why it matters
Flashing in this range can trigger seizures. This is the one criterion that can cause physical harm.
What a failure looks like
- failsA strobing animation in an ad or a game.
- failsRapid video cuts between very light and very dark frames.
How to fix it
- doKeep flashes below three per second, or below the general flash and red flash thresholds.
How A11ySignal checks it
manual
No automated rule can judge this criterion, and we will not pretend otherwise. A clean scan does not mean 2.3.1 passes — it means nothing we can measure fails. This one needs a person.