Guideline 2.3: Do not design content in a way that is known to cause seizures.
Certain flashing screen effects can cause a seizure. Some people are not even aware that they can be effected by flashing until they are exposed to it. Web sites should be designed so that they do not cause seizure, by limiting flashing frequency to a range which is not harmful.
2.3.1: Three Flashes or Below Threshold (Level A)
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)
Content on the screen should not flash more than three times a second. When there are several flashing objects on the screen, for the purpose of calculation they add up. Also, you need to make sure that flashing is below the general flash and red flash thresholds. This is actually a rather complex calculation and currently some tools are under development to check conformance for this criterion. The best thing to do at this point is to eliminate anything which flashes more than three times a second, which will be Level AAA conformance.
While other elements on the screen may flash as the result of a browser or operating system, these can be adjusted so they do not have to be considered when developing a web page.
2.3.2: Three Flashes (Level AAA)
Web pages do not contain anything that flashes more than three times in any one second period. (Level AAA)
It is very similar to 2.3.1 without allowing any exceptions from the less than three flashes a second rule.