A Seizure Safety Review Measures the Three-Flashes Threshold

What We Keep Finding

Most accessibility rules protect access. This one prevents physical harm. Content flashing more than three times a second can trigger seizures in people with photosensitive epilepsy, with no warning to the person watching. Which is why this rule is written like an engineering specification rather than like guidance.

The specification has real numbers in it, and almost nobody publishes them, which makes most claims about this impossible to check. Content passes if either no more than three general flashes and no more than three red flashes happen in any one second, or the flashing area is small enough. Small enough has a figure, and it is the figure people get wrong. The limit is 25 percent of a 10 degree visual field. The 341 by 256 rectangle the standard quotes is that whole field rather than the share of it allowed to flash, so the number to hold is a quarter of its 87,296 pixels, roughly 21,824. It also rests on a stated screen size and viewing distance, so treat it as an estimate rather than a margin of safety. Those two routes are one test with two ways of passing, not a size loophole bolted onto a frequency rule.

The flashes themselves are defined too, and that is what makes a frame-by-frame measurement reproducible rather than a matter of opinion. A general flash is a pair of opposing changes in relative luminance of 10 percent or more, where the darker image is below 0.80. A red flash is any pair of opposing transitions involving a saturated red, which is a different kind of test rather than a stricter version of the same number. Red gets its own test because saturated red does something to the visual system the luminance measure does not capture.

This is one of a small handful of rules that reach everything on the page, including content nobody chose to rely on. So one flashing advertisement in a third-party slot stops your whole page conforming, at any level, however careful the rest of the site is. That is the hardest sentence on this page and it is why the ad slots get recorded along with everything else.

A specification can be measured, and that is what this review does. We record your pages and step through the recordings frame by frame, because a rule that guards against physical harm deserves measurement rather than a glance. And if everything passes, that is the finding we most want to hand you. Measured, dated, and settled.