Web Accessibility: Seizure Prevention on the Web

When designing websites it is imperative to follow the necessary standards to design a site that is safe for those who suffer from photosensitive epilepsy or other seizures.

Photosensitive epilepsy is a form of epilepsy in which seizures are triggered by visual stimuli that form patterns in time or space, such as flashing lights, bold, regular patterns, or regular moving patterns.

Web Content Accessibility Guidelines state that web pages should not contain anything that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds.

What is Excessive Stimuli and How to Avoid it

Excessive stimuli may be classified as any flashing or blinking text, objects, or other elements within a movie that flash or whose blink frequency is greater than three (3) times per second.

What else could trigger a seizure?

  • The intensity or contrast of flashes
  • Large Mouseovers
  • The color red
  • Excessive video elements containing flashes
  • Pages designed with a flicker frequency greater than 2 Hz and lower than 55 Hz

As a web developer you may be asked to push the creative boundaries for your client and perhaps one day you might be asking yourself “Is this site going to be safe and accessible for everyone?” If you feel you are crossing the threshold on the amount of flashing objects on your site or the intensity and regularity of your flashes seems excessive, you may now access a Photosensitive Epilepsy Analysis Tool via Trace R&D Center at the University of Wisconsin-Madison.

The Trace Center’s Photosensitive Epilepsy Analysis Tool is a free, downloadable resource for developers to identify seizure risks in their web content and software. The evaluation used by PEAT is based on an analysis engine developed specifically for web and computer applications.

PEAT can help developers determine whether animations or video in their content are likely to cause seizures. Not all content needs to be evaluated by PEAT, but content that contains video or animation should be evaluated, especially if that content contains flashing or rapid transitions between light and dark background colors ie. excessive strobe-light effects.

For More Information on Seizure Prevention on the Web

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