Guideline 2.3: Be careful with flashing visual content

Do not design content in a way that is known to cause seizures.

Three-flash-in-a-second is the lower rate recommended to show flash content for people who suffer from seizures due to photosensitivity, specially for red lights. But still there are more sensitive people that react to lower rates, so the recommendation is to eliminate this type of visual content.
What is the difference between “blinking” and “fashing”?

blinking
content that distracts, and can be used for a short time as long as it stops or can be stopped by the user. If blinking occurs faster than 3 per second, it should be considered a flash.
flashing
content faster than 3 per second, and large and bright enough to cause a seizure The chance to turn the flash off is not an option since the seizure could occur faster than most users could turn it off.

How do you know if your content is flashing or blinking? Try the Photosensitive Epilepsy Analysis Tool.

In order to not interfere with other content of the webpage, these criteria applies for the whole page.

2.3.1 Three Flashes or Below Threshold Level A

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.

Compulsory

  • Ensure that no component of the content flashes more than three times in any 1-second period
  • Keep the flashing area small enough
  • Use a tool to ensure that content does not violate the general flash threshold or red flash threshold

Recommended

  • Reduce contrast for any flashing content
  • Avoid fully saturated reds for any flashing content
  • Reduce the number of flashes even if they do not violate thresholds
  • Provide a mechanism to suppress any flashing content before it begins
  • Slow down live material to avoid rapid flashes (as in flashbulbs)
  • Freeze the image momentarily if 3 flashes within one second are detected
  • Drop the contrast ratio if 3 flashes within one second are detected

2.3.2 Three Flashes Level AAA

Web pages do not contain anything that flashes more than three times in any one second period

Compulsory

  • Ensure that no component of the content flashes more than three times in any 1-second period

Recommended

They are the same techniques as 2.3.1

More info