Test flashing content against seizure thresholds
Some flashing can trigger a seizure, and you cannot tell by looking at it. G15 is the technique of running the content through an analysis tool before it goes out, and W3C counts it as sufficient for the three-flashes rule at Level A. The tool measures against two thresholds, a general flash one and a red flash one, and the second exists because saturated red behaves differently from every other color. If the content fails, it does not go up until it has been changed. Two shortcuts cover the simpler cases. Content flashing three times a second or less can be settled by counting, and content flashing in one small area can be settled by measuring the area. Everything else needs the tool, because too many variables move at once for a person to hold them all.
How we find it in an audit
Anything that flashes, strobes or cuts fast gets pulled out of the audit and run through a photosensitivity analyzer. PEAT, from the Trace Center, is the one we use and the one W3C links. Video, animation, page transitions and game footage are the usual candidates, and a hero video with rapid cuts is the regular surprise. The analysis report gets archived with the finding, because this is the one accessibility test an owner cannot repeat by looking at their own page.
How affected users experience it
This rule protects people who are not blind at all, which is worth saying plainly, because the harm here is a seizure rather than a barrier. Photosensitive epilepsy means a pattern of flashing can set one off, and a person can be photosensitive without knowing it until the day they find out. A warning on the page is not enough by itself either. People miss warnings, children may not be able to read them, and a video that autoplays in a feed gives nobody the chance to heed one.
Passes vs. fails
Passes
The same video goes through PEAT, gets re-edited until it clears both thresholds, and is archived with its analysis report.
Fails
A hero video with strobe-style cuts ships without ever being analyzed, so nobody knows what it measures.
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 G15: Using a tool to ensure that content does not violate the general flash threshold or red flash threshold. 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.