Keep any flashing area below seizure thresholds
Flashing is dangerous in proportion to how much of your central vision it fills, so G176 caps the area rather than the flash. W3C's calculation assumes a 1024 by 768 display, 15 to 17 inches across, viewed from 22 to 26 inches away, which puts a ten degree field of vision at roughly 341 by 256 pixels. The safe limit is 25 percent of that. Read the scope carefully. The figure covers a single flashing area with nothing else on screen flashing at the same time, and W3C's own test requires exactly that. This is a sufficient technique for 2.3.1 Three Flashes or Below Threshold at Level A. It also has a hole in it. Readers on lower-resolution screens, or who enlarge, or who sit close, are at higher risk, and for those cases W3C points to a separate technique that does not depend on screen size or viewing distance at all. There is a second formula for displays whose size and viewing distance you actually know, such as a kiosk or a foyer screen.
How we find it in an audit
Our reviewers identify anything flashing more than three times a second, in ads, video and animated effects, and measure the flashing region against the safe area. Analysis tools such as PEAT do the video work. Page scanners detect none of this, so it is a manual sweep of every animated and video element on the site.
How affected users experience it
Photosensitive epilepsy is rare and its consequences are not. A seizure triggered by a web page is a medical event, and unlike most accessibility failures the harm lands before the person can leave. Flashing also triggers migraine and nausea in people with vestibular disorders, and it makes reading impossible for many people with attention-related disabilities. Nobody has to be looking for it. They just have to load the page.
Passes vs. fails
Passes
The same promotion uses a static banner with a small 100 by 50 pixel animated badge, well under the safe-area limit. Better still, no strobing at all.
Fails
A full-width hero banner strobes white to red five times a second during a sale, filling much of the viewer's central vision.
Other ways to satisfy this rule
3 guides on this site are filed under 2.3.1 Three Flashes or Below Threshold. W3C lists this one as sufficient for that rule on its own. Implement it correctly, in a way your readers' software actually supports, and the rule is met.
This guide is our interpretation of W3C technique G176: Keeping the flashing area small enough. W3C publishes its techniques as guidance rather than as the standard, and says so on every one of them. The success criterion is what conformance is measured against, and a technique is one documented way to meet it.