2.3.1 Three Flashes or Below Threshold: 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. (Level A)
Note: Since any content that does not meet this success criterion can interfere with a user’s ability to use the whole page, all content on the Web page (whether it is used to meet other success criteria or not) must meet this success criterion.
This success criterion conveys that there mustn’t be any flashing content that flashes more than 3 times per second. In the past, flashing content has caused Epileptic seizure in users. So it is a best practice to avoid the flashing content totally & achieve the same design & functionality using other development techniques.
How do I know if the flash meets the criterion?
PEAT-Photosensitive Epilepsy Analysis Tool can be used to determine if the flashing content passes this check point.
An exception has been provided for this check point so that designers/developers who want to use flashing content can use with caution. Here is the exception:
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.