Section 508 Web Standards
(j) Pages shall be designed to avoid causing the screen to flicker with a frequency greater than 2 Hertz (cycles per second) and lower than 55 Hertz.
What does it mean?
Avoid animations, blinking and scrolling that could trigger a seizure or cause headaches for some people.
How to evaluate
No elements will be on the page, including blinking text, animation and changing colors, that flicker at a rate of 2 to 55 Hz (cycles per second). There is little risk of inducing an epileptic seizure or causing a headache. The elements are not distracting to the reader. Attributes such as Blink or Marquee are not used.
For more information
Design Resources