Some people are susceptible to seizures caused by strobing, flickering, or flashing effects. This kind of seizure is sometimes referred to as a photoepileptic seizure because it is caused by pulses of light (hence the prefix "photo") interacting with the eye's light-receptive neurons and the body's central nervous system.
Most web content is completely harmless to individuals with photoepileptic tendencies. Even most animations, videos, moving text, and Flash objects do not present any danger. However, some developers insist on dramatic effects of flashing or flickering lights and strobe-like effects. Science-fiction style Flash objects, horror movie previews, and cheap-looking banner ads are among the worst offenders. Maybe the creators of these effects think they're "cool," but they are also potentially dangerous. Developers should make every effort to ensure that their content does not have strobing, flickering, or flashing effects.
What Constitutes Strobing, Flickering or Flashing?
The Web Content Accessibility Guidelines outline specific thresholds for size, frequency (faster than three flashes per second), intensity or contrast of the flashes, and red color. In general, if the content flashes more than three times per second, is notably large (a small animating image would not cause a seizure), has bright contrast in the flashes, it may cause a seizure and should be avoided. Of note is that Section 508 prohibits flickering effects with a frequency greater than 2 Hz (flickers per second) and lower than 55 Hz. While very little content in web pages would meet the WCAG thresholds, online video sometimes presents special effects that do meet them.
Because of the potentially serious nature of seizures, developers should be extra careful to avoid any graphics, animations, movies, or other objects which have strobing, flickering, or flashing effects. Developers should also avoid graphics which may induce nausea or dizziness, or that may be distracting.