Checkpoint (J) states that web developers should avoid causing the screen to flicker. See the sections below for details.
People with photosensitive epilepsy can have seizures triggered by flickering or flashing in the 2 to 55 flashes per second (Hertz) range with a peak sensitivity at 20 flashes per second as well as quick changes from dark to light (like strobe lights).
Of course, we can't show you an example of an image or text blinking at 20 flashes per second or we could be accused of trying to bring on an seizure.
The point is be careful when designing animated .gif images, using Macromedia Flash or displaying other refreshing content.