Flashing content

Success criterion 2.3.1, Three Flashes or Below Threshold, is Level A and it is the one criterion in the standard written to prevent physical harm. Nothing on a page may flash more than three times in any one second, unless the flashing stays below the general flash and red flash thresholds. The risk is a seizure in a reader with photosensitive epilepsy, which is why this criterion has no user preference attached to it, no opt-in, and no severity discussion: three per second is the line, and 2.3.2, Three Flashes, at AAA, removes even the threshold exemption.

The thresholds are what make the rule usable rather than absolute. A flash is a pair of opposing changes in relative luminance, and the general threshold is crossed when that change is 10 percent or more of maximum relative luminance while the darker state sits below 0.80. Saturated red gets its own rule, because transitions to and from deep red are more provocative than the same change in any other hue. And there is a small safe area: flashing is exempt if the flashing region is no larger than roughly a quarter of a small central area of vision, which works out at about 21,824 pixels, a region around 341 by 256, at a typical viewing distance. Three numbers, then: how fast, how bright a swing, and how big.

The failures in ordinary product work are rarely deliberate. An autoplaying hero video with a strobe cut, a glitch effect looping in a background, a loading state that inverts, a theme that flips back and forth while a preference resolves, a scroll-driven animation that stutters into a flicker when a heavy page drops frames. Note that prefers reduced motion does not discharge this duty. A preference is a standing request from the readers who made it, and this criterion protects everyone, including the reader who has never touched that setting. A slow pulse animation is not in scope at all, because it is one cycle every second or two and a small swing at that, and where the movement is long-running rather than dangerous the applicable rule is pause, stop, hide.

The specimen above runs a flash faster than the count and stays lawful, which is the part worth seeing. Its lamp swings between white and a very light grey four times a second: past three, and still not a flash, because the swing is 13.7 percent of maximum while the darker state sits at 0.86, and a general flash needs both a swing over 10 percent and a darker state under 0.80. The lamp is also 6,720 pixels, which puts it inside the small safe area on its own. Two exemptions, either of them sufficient. The third control asks for twenty four a second at full contrast over a large region, and the specimen refuses that one: it holds a still frame and shows the numbers it would be failing. Explaining a seizure risk by producing one, unattended, on a loop, beside an article somebody is reading, is the single thing this entry must not do, and the way out of that bind is not silence but measurement.