Three Flashes Explained
Some people are so sensitive that it is not possible to completely prevent them from having seizures. However, by eliminating all flashing between three and 50 flashes per second everywhere on the screen, the chances of a person having a seizure are reduced.
SC 2.3.1 Three Flashes or Below Threshold (Level A) allows flashing if it is dim enough or is confined to a small enough area. SC 2.3.2 (Level AAA) does not allow flashing at all between three and 50 flashes per second, regardless of brightness or size.
Even a single flashing pixel violates this criterion. The intent is to guard against flashing areas that are larger than a single pixel; but, since an individual may require magnification or high contrast settings, the prohibition is against any flashing.
Animation from Interactions Explained
While SC 2.2.2 Pause, Stop, Hide (Level A) addresses animation that is part of web content, SC 2.3.3 Animation from Interactions (Level AAA) addresses cases where movement occurs on the screen as a result of the user interacting with the content.
People with vestibular disorders, which affect eye movement and balance control, may become dizzy (vertigo) or experience nausea or headaches when encountering content of significant size moving across their visual field. Rapidly paging through an ebook mimics the effect. A similar effect is motion sickness caused by looking out a side window of a moving car. Effects can be quite severe, potentially causing vomiting and requiring bed rest to reduce the symptoms.
As a result, any user-initiated animation should be avoided, or users should be provided with a way to disable such animations.
This success criteria does not apply to essential user-initiated animation. Scrolling, for example, is essential. In this case, users are in control and can adjust their scrolling speed to avoid any motion-related symptoms.
One form of motion in web content that is known to initiate vestibular dysfunction is parallax scrolling. This occurs when, while scrolling, the foreground and background scroll at different rates. If there is a significant difference in the scrolling rate, vestibular symptoms can be brought on.