Three Flashes or Below Threshold exists to prevent seizures triggered by photosensitive epilepsy. It prohibits any flashing content, animations, videos, or rapidly strobing effects, from flashing more than three times in any one-second period, unless the flashing area is small enough and low-contrast enough to fall under the defined safe threshold.
This is treated as an absolute requirement rather than a design preference, since a failure can cause real physical harm. Video content, animated ads, and rapid loading spinners are the usual places this gets checked.