WCAG 2.2 · Guideline 2.3 · Operable
Seizures and Physical Reactions
Nothing on the page may flash or move in a way that can hurt somebody.
This is the guideline where a failure puts a reader in hospital. Content that flashes more than three times a second can trigger a seizure in somebody with photosensitive epilepsy, and most people who have it find out on the day it happens. A warning banner is not the fix. Warnings get missed, especially by children who may not be able to read them, and a seizure arrives faster than anybody can reach for a control. The name has a second half as well. Physical reactions covers motion that brings on nausea or migraine, which is a different hazard with a different answer.
One A/AA success criterion lives here, 2.3.1, and it carries a hard number. Three flashes in any one second is the limit, general or red, and content only escapes it when the flashing area stays under a quarter of the reader's central vision, which on W3C's own worked assumptions is about 21,824 square pixels. That is a quarter of a 341 by 256 block, not the block itself, and the numbers rest on a screen size and a viewing distance rather than holding everywhere. Two more criteria sit at Level AAA, one dropping that small-area escape and one covering motion the reader sets off. Nothing automated checks any of the three. Guideline 2.3 is the only guideline in WCAG with no automated test rule against a single one of its criteria, so a clean scan here means the guideline was never looked at. And 2.3 is not itself the rule. 2.3.1 is, and it is the one an auditor marks.
- Rules
- 1
- Level A
- 1
- Level AA
- 0
- Human testing only
- 0
The Seizures and Physical Reactions rule
Level AAA in this guideline
2 enhanced criteria sit under 2.3. They are outside the level almost every law names, so our audits treat them as reference rather than scope. Some are still worth adopting, and the Level AAA hub says which.
- 2.3.2Level AAAThree FlashesNothing on the page may flash more than three times in any one second.Not graded1 fixes
- 2.3.3Level AAAAnimation from InteractionsMotion an interaction sets off has to be possible to turn off, unless the animation is essential to the functionality or to the information it carries.Not graded2 fixes
What goes wrong here
These are the failures we find repeatedly under 2.3, across sites of every size.
- Video contains rapid flashing that nobody ever measured. It was placed there years ago, it looked fine to whoever placed it, and looking fine is not the test.
- Large parallax and full-page zoom transitions run on scroll. For a reader with a vestibular disorder that is nausea, and for a reader with migraine it is a trigger.
- Motion the reader sets off ignores the reduced-motion preference their operating system already carries. Motion the page starts by itself belongs to guideline 2.2 at Level A instead, which is the stricter home of the two.
- A loader or an alert state strobes and was never assessed. The label on the component tells you nothing here, so a spinner can be fine and a subtle pulse can fail.
Who it affects
- People with photosensitive epilepsy. This is the one guideline where a failure can trigger a seizure, which is why it gets fixed on the day it is found rather than scheduled.
- People with vestibular disorders, who get dizziness and nausea from large parallax and zoom effects.
- People who get migraines, for whom flashing and motion are common triggers.
How to work through it
- 1Find everything that flashes, strobes or oscillates, and note where it plays full-screen or scaled up, because the area is part of the test.
- 2Measure it. Judging by eye is how this one gets missed. Nothing may flash more than three times in one second unless it stays under both the general and the red flash thresholds, and the area threshold is a quarter of a ten-degree visual field, which works out at roughly 21,824 square pixels on the reference display the calculation assumes. That number moves with screen size and viewing distance, so treat it as a working figure rather than a guarantee.
- 3Check the loop boundary. Content can sit under the threshold through its run and break it at the point where it starts over.
- 4Send anything borderline for proper analysis. Specialist flash-analysis tools exist for exactly this, and no general-purpose accessibility scanner has a rule anywhere in this guideline.
- 5Never test by exposure. Do not ask a photosensitive person to watch something and report what happens, and do not do it yourself either.
- 6For motion, set the operating system to reduce motion, reload, and see what the page does. This is Level AAA, and honouring the preference is one sufficient route rather than the only one.
How the levels build
Level A sets the flash limit and covers the dangerous failures. Level AA adds nothing, which makes 2.3 the second guideline where the AA line changes nothing. Level AAA adds two. One removes the below-threshold escape entirely, so nothing may flash more than three times a second at any intensity or size. The other covers motion the reader sets off, and it has three routes through it: do not animate on interaction, offer a site-wide control, or honour the reduced-motion setting the operating system already carries. Motion essential to the activity is excepted from that one. Passing all three is not a medical guarantee, and no part of this guideline claims to be.
Other Operable guidelines
- 2.1Keyboard AccessibleEvery task has to be finishable from the keyboard alone.
- 2.2Enough TimeTime limits and moving content have to be under the reader's control.
- 2.4NavigableReaders have to be able to find their way around and know where they are.
- 2.5Input ModalitiesTouch, pointer, voice, and motion all have to work, not only the keyboard.
Part of the Operable principle · browse by level: Level A · Level AA · or the full 55-rule library.
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