Keep flashes to three per second maximum
Three flashes in any one second, and no more. G19 is the counting route through the seizure rules, and W3C counts it as sufficient for both of them, the Level A rule and the Level AAA one. That is a rare free upgrade, because one behavior clears the higher bar with no extra work. It is deliberately stricter than the Level A rule and far easier to check, which is the trade it offers you. The rule's own thresholds begin at three and a half flashes a second, so staying under three means never needing the complicated measurement at all. One condition rides along with it. Where there are three flashes, the content has to finish the second in the same light or dark state it started in.
How we find it in an audit
Counting is the method, frame by frame where it comes to that. Anything that flashes gets watched at reduced speed, so the answer is a count inside a one-second window rather than an impression of speed. Most content does not flash, so the list stays short and the check stays quick. Where content flashes faster than three times a second we stop counting and send it to an analysis tool, because past that point the size, the area and the color all start to matter together.
How affected users experience it
Photosensitive epilepsy gives no warning. A person can go years without knowing they have it, and find out from a page that flashed. That is why the rule is written as a hard limit rather than a caution, and why the limit sits well below where the risk actually begins. Staying under three flashes a second also spares readers with vestibular disorders and migraine, who get nausea and headache rather than seizures from the same content, and who have no way to tell in advance which page will do it to them.
Passes vs. fails
Passes
The banner pulses once or twice a second at most, and finishes the second in the state it started in.
Fails
An attention-grabbing banner strobes its background five times a second.
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 G19: Ensuring that no component of the content flashes more than three times in any 1-second period. 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.