Web Content and Prevention of Photosensitive Seizures

One of the web accessibility guidelines is that content that flashes or blinks should not blink more than three times within one second or with a frequency of more than 50 times. This guideline is specifically intended to provide a safe environment for individuals who may experience photosensitive seizures, such as those with photosensitive epilepsy.

This is specified in the Web Content Accessibility Guidelines (WCAG 2.0), which include the "1.4.3 Flashes" criterion, aimed at considering individuals who may be at risk of seizures due to flashing or flickering content.

This guideline appears to have been influenced by the Pokémon Shock incident. Pokémon Shock occurred on December 16, 1997, when some children who watched episode 38 of the show experienced seizures, vomiting, and dizziness, and were taken to the hospital. Most of the patients transported were children ranging from infants to teenagers, and according to TV Tokyo, a total of 750 people were affected, with 135 requiring hospitalization. An investigation at the time found that rapid flashing of red and blue lights at a rate of 12 flashes per second for 4.5 seconds led to photosensitive seizures. As a result, many media outlets became more cautious about flashing lights and visual changes in their content.

There are also tools available to analyze whether visual stimuli in web content or software could trigger seizures, one of which is the Photosensitive Epilepsy Analysis Tool (PEAT).