Epilepsy is the fourth most common neurological disorder, with approximately 50 million people worldwide who are living with epilepsy. In fact, there are more people living with epilepsy than with autism spectrum disorders, cerebral palsy, Parkinson’s disease, and multiple sclerosis – combined. There are special considerations developers should keep in mind, as some content may actually be dangerous to people who have epilepsy.
Epilepsy and Seizures
Epilepsy is a neurological condition that affects the nervous system and it is known as a seizure disorder. Seizures are caused by disturbances in the electrical activity of the brain and may be related to a brain injury or family tendency. However, the cause is often unknown.
Photosensitive epilepsy is a type of epilepsy in which most (or all) seizures are triggered by flickering or flashing light, whether natural or artificial light. Photo epileptic seizures can also be caused by overexposure to video games and high-contrast patterns, such as stripes or checks.
Web Accessibility for Epilepsy
Most web content is completely harmless to individuals with photosensitive epilepsy. However, some developers insist on using dramatic effects like flashing or flickering lights and strobe-like effects, which may be dangerous to some users. Even if the object does not trigger a seizure, it could be distracting to users and may even cause nausea or dizziness in some people. This is definitely not a feeling you want users to associate with your digital content.
The World Wide Web Consortium (W3C) has included guidance related to seizures in its Web Content Accessibility Guidelines (WCAG). The W3C has provided success criterion that is intended to allow users to access all web content without seizures being induced due to photosensitivity. They recommend a “three flashes or below threshold” and suggest that developers incorporate the following techniques to ensure criterion is met.
- Ensure that no component of any content flashes more than three times in any 1-second time period
- Keep any flashing areas small, generally to an area approximately 341 by 256 pixels or less
- Use a tool to ensure that content doesn’t violate the flash thresholds
- Reduce contrast for any flashing content
- Avoid using fully saturated reds for any flashing content
- Reduce the number of flashes, even if they don’t violate the flash thresholds
- Provide a mechanism to suppress any flashing content before it begins
- Slow down live material in order to avoid rapid flashes
- Freeze the image momentarily if three flashes are detected within one second
- Drop the contrast ratio if three flashes are detected within one second
- Allow users to set a custom flash rate limit
Because of the potentially serious nature of seizures, developers should make every effort to ensure that their content does not have strobing, flickering, or flashing effects. Developers should also avoid graphics which may induce nausea or dizziness, or that may be distracting.
Further reading from WeCo’s Accessibility Blog about epilepsy and web accessibility:
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