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2.3.1
Web pages do 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.
Always
Check if anything flashes more than 3 times per second. This is very difficult to test, so you might rely on making assumptions rather than testing it properly.
Just looking at the page can give you an indication if something is flickering, but it can be inadequate especially if the flickering is close to the threshold.
If in doubt, err on the side of caution and fail it (include the caveat that you haven't properly tested it) because this can potentially be life-threatening. Warning users of the flashing content in advance is not enough to pass this success criterion.
You can also consider checking through anything listed under "other ways to test" below.
It's not just flashing content which can cause seizures. Static images can cause seizures as well.
Mozilla has documentation on web accessibility for seizure disorders. It includes a section on patterns and parallax with a list of things to look out for in static images, for example an unchanging dark and light pattern containing 8 or more lines.
Ofcom have further information on potentially harmful patterns in their guide on Harm and Offence (see Annex 1 in the guidance notes).
It's best to not use any flickering content at all. If it is used, it should be switched off by default and be controllable by the user (which should be checked under 2.2.2 Pause, Stop, Hide).
There are tools that can test flickering frequencies. However, these are often not practical to use in an accessibility test as they either cost money, are old, or need special file formats.
- Harding Test - paid service
- HardingFPA - paid service
- OnlineFlashTest.com - paid service, requires specific file formats
- Trace Center's Photosensitive Epilepsy Analysis Tool (PEAT) - free, old, requires specific file formats, Windows only
If you see a potentially noncompliant video, you might consider making a business case for buying one of those services.
If a video or animation looks like it might be a fail, consider asking the supplier if they have tested it and whether they have a certificate that shows the result of the test.
If this Success Criteria fails it means the whole page fails because there is something interfering with the other content in a way which makes it inaccessible to some people. That includes if third-party content causes this to fail, even if it is otherwise excluded from the audit. See Non-Interference.
No difference