Flash threshold

Meaning

"Flashing" refers to content that can trigger a seizure (if it is more than 3 per second and large and bright enough). A safe flash threshold is to ensure that material does not flash more than 3 times per second. The color of the flash matters as well, because people tend to be more sensitive to red flashing than to other colors.

Background

This Web Content Accessibility Guideline (WCAG) recommendation is one that probably has the most research behind it because it stems from the days of broadcast TV.

People with photosensitive seizure disorders can have a seizure triggered by content that flashes at certain frequencies for more than a few flashes. Any content that does not meet this success criterion can interfere with a user's ability to use the whole page.

If you can’t avoid flashing animations providing advanced warning of flashing content and an alternate version of the content without the flashing effect (like a text-only image description) is a good thing to do.

Alternatives

Additional information on photosensitivity seizures can be found here: https://theweco.com/websites-seizures-developing-accessibility/

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