This criterion ensures that content does not include flashing that could trigger seizures or other serious physical reactions. Certain visual patterns, especially rapid flashes or high-contrast colour changes, can cause seizures, migraines, dizziness, or nausea for some users.
Because these effects can occur almost immediately and without warning, this requirement applies to all content on the page, not just primary features or interactive elements.
WCAG Success Criterion
2.3.1 Three Flashes or Below Threshold (Level A)
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.
Note:
Since any content that does not meet this criterion can interfere with a user’s ability to use the entire page, all content must meet this requirement, including decorative or background elements. This is covered by Conformance Requirement 5: Non-Interference.
What WCAG Means by a “Flash”
A flash is a rapid change in brightness or colour that happens faster than the eye can comfortably process.
In practical terms, a flash occurs when content rapidly alternates between:
- Light and dark, or
- One colour and another (especially red)
If this happens more than three times per second, and the flashing is large or intense enough, it can trigger seizures.
Why This Matters
Some individuals have photosensitive seizure disorders, including photosensitive epilepsy. For these users, exposure to certain flashing patterns can trigger seizures within seconds.
Even users without epilepsy may experience:
- Migraines
- Dizziness or nausea
- Eye strain or disorientation
Because seizures can occur faster than a user can react, warning messages or “turn it off” controls are not sufficient when flashing exceeds safe thresholds.
Intent of the Criterion
This success criterion intends to ensure that content itself does not introduce visual flashing patterns known to trigger seizures.
Authors cannot control:
- Display hardware
- Operating system rendering
- Individual viewing distance
They can control whether the content they provide includes flashing that exceeds established safety thresholds.
This criterion focuses on prevention at the source, not mitigation after exposure.
Flashing vs. Blinking
These terms are often confused, but are not interchangeable in WCAG.
Blinking
- Slow on/off visual changes
- Primarily causes distraction
- Allowed if it stops within a short time or can be paused
Flashing
- Rapid brightness or color changes
- Can trigger seizures
- Must never exceed three flashes per second, even briefly
If blinking occurs faster than three times per second, it is treated as flashing and falls under this criterion.
Scope of the Requirement
This criterion applies to all visual content, including:
- Animations
- Videos
- GIFs
- Canvas or SVG effects
- CSS or JavaScript-driven visual changes
Flashing may be caused by:
- Strobe effects
- Rapid explosions or muzzle flashes
- Lightning effects
- Alternating high-contrast frames
Content must be evaluated at the largest size at which it may be viewed, including:
- Full-screen playback
- Zoomed or magnified views
Red Flash Sensitivity
People are particularly sensitive to saturated red flashing. WCAG includes a stricter test for red flashes because long-wavelength, high-contrast red transitions are more likely to trigger seizures than other colours.
Even if flashing frequency appears low, saturated red transitions may still exceed safe thresholds.
Who This Helps
This criterion primarily supports users who:
- Have photosensitive epilepsy
- Have other seizure disorders
- Experience migraines triggered by visual stimuli
- Are sensitive to intense contrast changes
Meeting this requirement makes content safer for everyone, without limiting access to the full experience.
Common Failure Patterns
Failures often occur when:
- Animated GIFs loop rapidly without testing
- Video effects include strobe-like flashes
- CSS animations alternate brightness too quickly
- Games or promotional effects use flashing to draw attention
These failures can exist even when flashing is not the main focus of the content.
Examples
Passing example
A video showing lightning limits flashes to no more than three per second, even during intense scenes.
Passing example
A rapidly changing animation occupies a sufficiently small portion of the user’s visual field to remain below WCAG’s general flash threshold, even at maximum zoom or full-screen display.
Failing example
A full-screen animation alternates between bright white and dark red more than three times per second.
Rule Description
Content must not include flashing that exceeds three flashes per second, unless it remains below the general flash and red flash thresholds defined by WCAG.
This requirement applies regardless of whether the flashing is decorative, informational, or interactive.
How to Tell If Your Content Passes (Without Math)
- Identify any content that rapidly changes brightness or colour
- Check whether changes occur more than three times per second
- Evaluate the size of the flashing area at maximum viewing size
- Check for saturated red transitions
- If flashing exceeds thresholds → fail
- If flashing stays within thresholds → pass
How Developers Actually Test This
- Use the Photosensitive Epilepsy Analysis Tool (PEAT) to analyze video and animations
- Review animated GIFs frame-by-frame
- Test video content in full-screen mode
- Review CSS animations that alternate colour or brightness
Manual review is essential, automated accessibility tools do not reliably detect flash risks.
Wrapping Up
Three Flashes or Below Threshold is a non-negotiable safety requirement. Unlike many accessibility issues, seizure triggers cannot be mitigated with instructions, controls, or user choice. If flashing exceeds safe limits, harm can occur before users can respond.
Meeting this criterion ensures that content can be experienced safely, without excluding users with photosensitive conditions from the web.
