The law states:
"Pages shall be designed to avoid causing the screen flicker with a frequency greater than 2 Hz and lower than 55 Hz."
Flickering and flashing page elements can cause epileptic seizures in some viewers. They are also disruptive to many readers with cognitive disabilities or low vision.
If using animated images (graphics or text), you must keep the flickering (or color change) out of the 2Hz - 55Hz (Hz is a cycle per second) range. If it looks like too much, it probably is a problem. You also should not use the blink or marquee tag to make text blink or move.