Operable · 2.3 Seizures and Physical Reactions
Nothing on a page may flash more than three times in any one-second period, unless the flashing stays below the general flash and red flash thresholds, which depend on the flashing area, the size of the luminance change and whether saturated red is involved.
Live demo
The same task, done two ways: switch between the version that passes and the one that fails, or put them side by side. Everything in the box is live, so try it with a keyboard, a pointer or a screen reader.
Compare the one-second strips for both versions. Nothing here flashes: each strip is a still chart of what the badge would do.
A sale badge about 100 by 60 pixels switches between saturated red and black five times a second.
Viewed at 1024 by 768, it covers about 7 percent of a 10 degree field of vision.
A sale badge about 200 by 150 pixels switches between saturated red and black five times a second.
Viewed at 1024 by 768, it covers about a third of a 10 degree field of vision.
The tools follow the version you last used.
What happened
Why it passes
The badge still flashes five times a second, but it covers only about 7 percent of a 10 degree field, under the 25 percent limit, so it stays below both thresholds. Slowing it to three flashes a second or fewer would be safer still.
Why it fails
Five saturated red flashes a second over about a third of a 10 degree field cross both the general and the red flash thresholds, which can trigger a seizure.
Examples
Three examples, each one running: the usual ways this criterion fails, and code that passes. Listen to them, measure them, or copy the code.
Fails
Live preview
.sale-banner { min-height: 40vh; background: #ff0000; animation: sale-flash 0.2s infinite;}@keyframes sale-flash { 0%, 49% { background: #ff0000; } 50%, 100% { background: #000000; }}Each 0.2 second cycle is one flash, so this large banner flashes five times a second in saturated red. That crosses both the general and the red flash thresholds.
Passes
Live preview
.sale-banner { min-height: 40vh; background: #ff0000; animation: sale-flash 1s 3;}@keyframes sale-flash { 0%, 49% { background: #ff0000; } 50%, 100% { background: #000000; }}One flash a second, stopping after three, stays within the three-flash limit at any size and in any color (technique G19).
Passes
Live preview
/* 16 by 16 pixels; a 10 degree field is about 341 by 256 pixels at 1024 by 768 */.live-dot { width: 16px; height: 16px; border-radius: 50%; animation: live-blink 0.25s 8;}@keyframes live-blink { 0%, 49% { background: #ffffff; } 50%, 100% { background: #1f2937; }}Four flashes a second is more than three, but the dot covers well under 25 percent of any 10 degree field, so it stays below the general flash threshold (technique G176). It would still fail 2.3.2.
Why it matters
Flashing can trigger seizures in people with photosensitive epilepsy, and large, bright or saturated red flashes carry the greatest risk. A seizure can occur before the user has any chance to look away.
W3C: Understanding 2.3.1Level A
Level A is the minimum: failing it can shut some people out completely.
Where it is required
Required where the law names WCAG 2.0 or later at this level: the United States, Canada, the European Union, the United Kingdom, France, Germany, Italy, Spain, the Netherlands, Ireland, Norway, Switzerland, India, Japan, Australia, New Zealand and Israel.
How to test
A few concrete steps. Automated tools find some failures; most need a person.
Common failures