Three Flashes or Below Threshold
Web pages must not contain anything that flashes more than three times in any one-second period, unless the flashing is below specific brightness thresholds.
What it requires
WCAG 2.0 SC 2.3.1 protects users with photosensitive epilepsy and migraine sensitivity. Flashing content that exceeds three flashes per second in a sufficiently bright/contrasting region can trigger seizures.
The threshold is technical: the flashing must stay below 25% screen area or below specific luminance/red-flash thresholds. In practice, ecommerce sites should avoid flashing content entirely except for very small UI accents.
Common Shopify failure
Promotional carousel that auto-cycles every 0.3 seconds with high-contrast color shifts. Flash-sale banner with rapid red/yellow color flashing.
How to fix it
AccessComply detects animation rules with sub-300ms iteration cycles on background-color or filter properties and flags them for merchant review. Auto-fix slows the iteration to a safe 1.5s minimum.
Primary source: W3C — WCAG 2.0 Understanding 2.3.1