Goal
Prevent seizures caused by flashing content.
Success Criterion · WCAG 2.3.1
Three Flashes or Below Threshold
Web pages do not contain anything that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds.
Level AWCAG 2.0Operable2.3 · Seizures and Physical Reactions
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What to do
Do not include content that flashes more than 3 times per second above the general flash and red flash thresholds.
Why it matters
Flashing content can trigger seizures in people with photosensitive epilepsy.
Success criterion
What WCAG 2.3.1 requires
Summarized directly from the official Understanding document so teams can quote the requirement accurately.
Web pages do not contain anything that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds.
Intent
Why WCAG created this requirement
- Flashing can cause seizures; prevention is a safety requirement.
- Both frequency and intensity matter (thresholds exist for general and red flashes).
- Applies to video, animated GIFs, CSS/JS animations, and interactive effects.
Benefits
Who gains when you pass
- People with photosensitive epilepsy are protected from seizure triggers.
- Users sensitive to flashing avoid discomfort and nausea.
- Everyone benefits from avoiding aggressive flashing effects.
Why it matters
User impact when this criterion fails
Summaries drawn from the Understanding document help you socialize impact statements with product stakeholders.
Seizure risk is a severe harm; users may avoid the site entirely.
Flashing effects can cause headaches, dizziness, or nausea.
Overview
Avoid dangerous flashing. Content must not flash more than three times per second unless it is below established thresholds for overall luminance change and red flashing. This protects users with photosensitive conditions.
- Avoid strobe-like patterns, rapid full-screen flashes, and high-contrast blinking.
- Be cautious with red flashes; red is a known risk factor.
- If flashing is part of media, provide a non-flashing alternative version.
- Test content with established tools for flash thresholds where applicable.
Reference: All summaries and highlights originate from Understanding WCAG 2.3.1 and the W3C quick reference.
Fast facts
- Conformance level
- Level A
- WCAG version introduced
- WCAG 2.0
- Principle
- Operable
- Guideline
- 2.3 · Seizures and Physical Reactions
Examples
Make success tangible for teams
Share pass/fail snapshots to coach designers, engineers, QA, and content authors.
Animated banner
Pass
Smooth fade transition at low frequency with no rapid flashing.
Fail
High-contrast banner rapidly blinks on/off multiple times per second.
Video effect
Pass
Edited video removes strobe effect or reduces flash intensity below thresholds.
Fail
Strobe lighting effect flashes rapidly throughout the clip.
Evidence to keep
Document conformance decisions
Capture artifacts for VPATs, procurement reviews, and regression testing.
- Document an “anti-flash” standard for design and motion guidelines.
- Record verification of media assets using PEAT or similar tools where applicable.
Official resources
Deep dives and supporting material
Keep these links handy when writing acceptance criteria or responding to audits.
Understanding 2.3.1
Official W3C interpretation, techniques, and intent for Three Flashes or Below Threshold.
Quick Reference 2.3.1
Filterable list of sufficient techniques and failures.
Technique G19
Ensuring that no component flashes more than three times per second.
Technique G176
Keeping flashing areas small enough and below thresholds.
Implementation checklist
Capture progress and blockers
- Audit animations and media for any flashing behavior.
- Remove or redesign flashing effects to stay under 3 flashes per second.
- Avoid full-screen flashes and high-contrast blinking transitions.
- Review third-party embeds (ads, widgets) for flashing content.
- Provide safer alternatives for any legacy flashing assets.
Testing ideas
Prove conformance with evidence
- Review pages for flashing elements in animations, video, and interactive effects.
- Count flashes per second during animations; ensure ≤ 3 per second unless below thresholds.
- Use a photosensitive epilepsy analysis tool (PEAT) or equivalent to test media when appropriate.
- Check for red flashing patterns specifically.
- Test in different themes/backgrounds to ensure contrast changes don’t introduce flashing.
Related success criteria