‘Three flashes or below threshold’ requires that no content flashes more than three times per second.
Flashing content on a website can cause difficulties for users with photosensitive seizure disorders such as epilepsy. Flashing content can cause these users to suffer a seizure.
How to Pass ‘Three Flashes or Below Threshold’
Don’t add anything to your website that flashes more than three times per second.
There’s one exception to this guideline based on the size of the flashing content, but I recommend you ignore it and just don’t let anything flash more than three times per second.
“the combined area of flashes occurring concurrently occupies no more than a total of .006 steradians within any 10 degree visual field on the screen (25% of any 10 degree visual field on the screen) at typical viewing distance.”Understating Success Criterion 2.3.1
If you can understand that you’re smarter than me!
‘Three Flashes or Below Threshold’ Tips
Remember, flashing is different to blinking (see Pause, Stop, Hide). Blinking can distract users but doesn’t cause seizures.
If blinking content occurs three times per second, it is considered flashing content.
The exception is removed at Level AAA in Three Flashes.