People With Photosensitive Epilepsy
The smallest affected group and the highest-severity consequence on this entire list. There is no partial credit here.
This is the one accessibility failure that can send someone to the hospital.
There is no graceful degradation. Content that flashes more than three times per second in a large enough area, particularly with saturated red, can trigger a seizure. This has happened deliberately in documented online attacks, which is part of why it is a Level A criterion with no exceptions for decorative intent.
Scale: Photosensitivity affects roughly 3% of people with epilepsy, and epilepsy itself affects a little over 1% of the U.S. population. (Source: CDC / Epilepsy Foundation. Survey-based disability figures undercount — treat this as a floor.)
What they're using
- Extensions that block animation and autoplay
- Reduce Motion and reduced-transparency settings
- Mostly avoidance of unfamiliar media
3 barriers we find most often, and the criteria behind them.
Content flashing more than three times per second
Video, GIFs, transitions, loading animations, and ads. The general flash and red flash thresholds in WCAG 2.3.1 are the test, and this is Level A.
Success criteria: 2.3.1 Three Flashes or Below Threshold A
Third-party and user-generated media you did not author
Ad networks, embedded video, and uploads. You are responsible for what your page renders, including what a vendor injects into it.
Success criteria: 2.3.1 Three Flashes or Below Threshold A
Rapid strobing transitions
High-contrast flashes between slides or during loading, which are easy to introduce without anyone thinking of it as flashing content.
Success criteria: 2.3.1 Three Flashes or Below Threshold A · 2.2.2 Pause, Stop, Hide A
The changes that actually remove the barrier.
Analyze video before publishing
The Photosensitive Epilepsy Analysis Tool (PEAT) from the Trace Center is free and is the standard check.
Put flash limits in vendor and ad contracts
If you cannot inspect every asset, make compliance a contractual obligation with a takedown path.
Warn before autoplaying unreviewed media
For user-generated content, a click-to-play poster frame keeps unreviewed video from running on load.
Keep loading and transition effects gentle
No strobe, no rapid inversion, no saturated red flashes.
Three tests you can run before you call anyone.
- Run any video with rapid cuts or effects through PEAT before it goes live.
- Audit what your ad network is allowed to serve.
- Check loading spinners and page transitions for high-contrast flicker.
If any of those go badly, you have your answer and you did not need a vendor to get it. What a vendor is for is the part after: knowing which of the 55 criteria that maps to, how deep it goes, and what it costs to fix properly rather than patch.
Run the Free CheckerWhere this shows up in practice
Other people the standard was written for.
Older Adults
Aging rarely produces one disability. It produces slightly reduced vision, hearing, dexterity, and processing speed simultaneously — which compounds.
Read the profileSituational and Temporary Barriers
A broken arm, bright sunlight, a noisy train, a bad connection, a borrowed laptop. The accommodation is identical to the permanent one.
Read the profileAll 55 Success Criteria
Every Level A and AA criterion in plain language, with the failure we see most often for each.
Reference