The overarching accessibility requirement for flashing content is that if it is not below the WCAG general flash and red flash thresholds , then within a one second period it must not flash more than three times.
The following information details the WCAG general flash and red flash thresholds -
"general flash and red flash thresholds - a flash or rapidly changing image sequence is below the threshold (i.e., content passes) if any of the following are true:
- there are no more than three general flashes and / or no more than three red flashes within any one-second period; or
- the combined area of flashes occurring concurrently occupies no more than a total of .006 steradians within any 10 degree visual field on the screen (25% of any 10 degree visual field on the screen) at typical viewing distance"
So, an example of passing content would be an image sequence including two general or red flashes in one second. Or, if there are more than three general or red flashes in one second, the content would still pass if the combines area did not exceed the WCAG threshold of "25% of any 10 degree visual field on the screen."
Additionally, it is important to understand what WCAG defines as a "general flash" and a "red flash":
- "A general flash is defined as a pair of opposing changes in relative luminance of 10% or more of the maximum relative luminance where the relative luminance of the darker image is below 0.80; and where "a pair of opposing changes" is an increase followed by a decrease, or a decrease followed by an increase, and
- A red flash is defined as any pair of opposing transitions involving a saturated red.
Exception: Flashing that is a fine, balanced, pattern such as white noise or an alternating checkerboard pattern with "squares" smaller than 0.1 degree (of visual field at typical viewing distance) on a side does not violate the thresholds."
You can use the Photosensitive Epilepsy Analysis Tool (PEAT) to test flashing content and ensure it meets the WCAG guidelines.