The Three-Flash Threshold for Seizures
Keep flashing content within safe limits.
The Three-Flash Threshold for Seizures is a free Web Accessibility Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Web Accessibility Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Flashing Can Harm
For some people, flashing light on a screen can trigger a seizure. This is one place where an animation choice becomes a safety choice. ⚠️
Photosensitive Epilepsy
The risk has a name: photosensitive epilepsy. Bright, rapid flashes are the classic trigger for people who live with it.
The WCAG Rule
WCAG sets a clear line. Nothing should flash more than three times in any one second, or it must stay below a small safe area.
Counting a Flash
A flash is a pair of opposite changes: bright to dark and back. Three of those bright-dark cycles within a second crosses the threshold.
Red Is Extra Risky
Saturated red flashes carry their own warning in WCAG, because deep red transitions are especially likely to provoke a seizure.
Size Matters
A tiny flashing dot is low risk; a flashing full screen is high risk. The rule cares about how much of the view is involved.
Watch Auto-Play Video
The danger is not just GIFs. Video with strobe effects, explosions, or rapid cuts can flash past the limit without you noticing.
Beware Rapid Loaders
Even a frantic loading spinner or a blinking alert can flash too fast. Keep any repeating pulse slow and gentle.
Test the Frequency
When in doubt, measure. The free PEAT tool from the Trace Center analyzes media for flash that breaks the threshold.
PEAT (Photosensitive Epilepsy Analysis Tool) checks media for unsafe flashingSafer by Default
The easiest fix is to avoid flashing at all. If an effect even feels strobe-like, slow it down well under three flashes per second.
Warn Before You Show
If flashing media is truly unavoidable, give a clear warning first so a sensitive viewer can choose to look away.
Quick Check
What is the safe flashing limit under WCAG?
Recap
Keep anything flashing under three flashes per second, watch saturated red, and check video with PEAT. Safer is slower. ✅
Frequently asked questions
Is the “The Three-Flash Threshold for Seizures” lesson free?
Yes — the full text of “The Three-Flash Threshold for Seizures” is free to read here on the web, and the Web Accessibility Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Web Accessibility Academy course, upgrade to CoddyKit PRO.
What will I learn in “The Three-Flash Threshold for Seizures”?
Keep flashing content within safe limits. You practise Web Accessibility Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Web Accessibility Academy?
No prior experience is required. Web Accessibility Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “The Three-Flash Threshold for Seizures” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Web Accessibility Academy lesson?
Yes. Every Web Accessibility Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- prefers-reduced-motion in CSS and JS
- Vestibular Disorders and Parallax Risks
- The Three-Flash Threshold for Seizures
- Pause, Stop, Hide for Auto-Moving Content