2.3.1 Three Flashes or Below Threshold

This criterion protects people with photosensitive epilepsy and other photosensitive conditions, for whom flashing content is not an annoyance but a medical hazard: content that flashes rapidly enough, over a large enough area, can trigger a seizure within seconds of appearing on screen. Unlike almost every other criterion, the harm happens before the user has any chance to react, so it is graded as a safety criterion. Treat any suspected fail as a blocker and escalate it, rather than logging it alongside routine findings.

The auditor's mental model has two parts. First, the bright line: nothing may flash more than three times in any one second period. Second, the escape hatch: content that does flash faster can still pass if it stays below the general flash and red flash thresholds, which conceptually cover flashing that occupies only a small area of the visual field or involves only a low change in brightness or contrast. Saturated red flashing has its own, stricter threshold because red light is a particularly potent seizure trigger. The thresholds are precise photometric definitions; in a manual audit you will rarely be able to compute them, so the working posture is conservative: if content visibly strobes and you cannot show it is below threshold, grade it as a fail.

Passes:

• A hero video with fast editorial cuts between scenes of similar brightness: cuts are not flashes unless they produce rapid opposing brightness changes.
• A notification dot that pulses twice per second: at or below three flashes in any one second period.
• A single simulated camera flash effect when the user captures a screenshot in-app: one flash, well under the limit.
• A tiny recording indicator that blinks rapidly: the flashing occupies a very small area of the visual field, so it sits below the general flash threshold.
• A subtle skeleton loader shimmer with a gentle brightness change: the contrast change is low, not a strobe.

Fails:

• A strobe sequence in an autoplaying full-viewport background video, flashing white many times per second: the classic case, and a blocker.
• A glitch-effect heading that alternates between light and dark states in rapid succession: decorative intent does not exempt it.
• A celebratory strobing overlay on order confirmation that flashes the whole screen faster than three times per second.
• A large alert region that flashes saturated red rapidly: red flashing has a stricter threshold and a large red strobe fails it.
• A loading state that flickers rapidly across most of the screen because of a re-render bug: unintentional flashing counts exactly like designed flashing.

Not a fail under this criterion:

• A smoothly scrolling ticker or auto-advancing carousel with no brightness strobing: moving content without flashing belongs under SC 2.2.2 Pause, Stop, Hide.
• Slow blinking content used to draw attention (for example, once per second): review it under SC 2.2.2 Pause, Stop, Hide, not here.
• A torch or camera-flash feature: hardware light output is not screen content.
• Motion effects such as parallax or zoom triggered by interaction with no flashing: consider SC 2.3.3 Animation from Interactions (Level AAA) as guidance instead.

• SC 2.2.2 Pause, Stop, Hide. That criterion covers moving, blinking, scrolling, and auto-updating content and asks for user control. Motion without flashing files there. Crucially, a pause control is not a cure for a 2.3.1 fail: a flashing animation that auto-plays and crosses the thresholds has already done its damage before the user can find the pause button. Grade it as a 2.3.1 fail even if it is pausable.
• SC 2.3.2 Three Flashes. The Level AAA sibling removes the threshold escape hatch entirely: any flashing above three per second fails, however small or dim. If a finding cites a tiny below-threshold flicker as a Level A fail, the auditor is applying the AAA rule.
• SC 2.3.3 Animation from Interactions. That Level AAA criterion is about motion animation (parallax, zooming, sliding) triggered by interaction, which affects people with vestibular disorders. Motion sickness findings go there; seizure-risk flashing goes here. The two harms and the two tests are different.
• Blinking versus flashing. Slow blinking to attract attention is a distraction issue under SC 2.2.2 Pause, Stop, Hide; flashing is rapid alternation posing a seizure risk under this criterion. The dividing line in practice is rate and abruptness: three or fewer flashes per second is at worst a blinking finding, more than three is a flashing one.

AUDITSU's audit walkthrough asks about flashing content on every screen you review: whether any video, animation, or transient effect flashes, whether anything exceeds three flashes in any one second period, and whether fast flashing could plausibly claim the small-area or low-contrast escape. Each question records a pass, fail, or not applicable result per screen with evidence attached, and because this is a safety criterion the walkthrough treats suspected fails as blockers rather than ordinary findings.

The walkthrough is deliberately conservative here. Precise threshold measurement needs specialist frame-by-frame video analysis, so where content visibly strobes and cannot be shown to be below threshold, the guidance is to grade it as a fail and recommend the flashing be removed or slowed, which is almost always the cheaper fix than proving compliance photometrically.

For the full guided workflow, see the audit platform.