Pages shall be designed to avoid causing the screen to flicker with a frequency greater than 2 Hz and lower than 55 Hz.

-- Consistent with WCAG 7.1   Note: The WC3 discourages use of the BLINK and MARQUEE tags in HTML.

Research shows that people with photosensitive epilepsy can experience seizures when exposed to flashing images between 4 to 59 hertz (flashes per second). 

Associated Press, July 21, 2004 – Researchers Study Pokemon Seizures

Japanese children who had seizures during a “Pokemon” cartoon in 1997 have generally not had another one unless they already had epilepsy, researchers say. The TV show sent at least 685 Japanese viewers, mostly children, to emergency rooms with symptoms ranging from nausea and hyperventilation to convulsions. They were apparently made sick by a scene with extremely rapid flashes of red and blue.