What do I need to know so that content does not cause seizures?
People who have epilepsy can have seizures when exposed to flashing or flickering lights. There are three causes of flickering lights on computer screens:
- Flashes can be caused by the display.
- Flashes can be caused by the computer and how it renders images and other content.
- Flashes can be caused by the content itself.
Although web authors have no control over the first two, they can ensure that flicker is not caused by the content, such as a movie of strobe flashes or an animation of rapid-fire explosions.
To conform at Level AA, WCAG 2.1 describes how to determine safe values for flashing content. To conform at Level AAA, a page must avoid all flashing content.
Three Flashes or Below Threshold Explained
People with a form of epilepsy called photosensitive seizure disorder have seizures when exposed to flashing or flickering lights. Many colours can cause these seizures, but flashing red lights are known to trigger seizures more readily than other colours, and the larger the area flashing, the greater the possibility of inducing a seizure.
To meet this requirement, web authors must ensure that flickering is not caused by the content itself, such as a movie of strobe flashes or an animation with rapidly switching colours.
SC 2.3.1 allows content to flash if it is dim enough and confined to a small area. W3C publishes formulas for determining safe values for “general flash” and “red flash” thresholds. But even these so-called safe values can trigger seizures. Research suggests the greatest sensitivity to flashing is between 10 hz and 25 hz. The next section, SC 2.3.2, is more stringent.