WCAG Deep Dive: 2.3 Seizures and Physical Reactions – Prioritizing User Safety
Posted: Mon Aug 25, 2025 10:05 am
Dear WCAG Plus Forum members,
Today's topic addresses one of the most critical aspects of web accessibility: user safety. The Guideline 2.3 Seizures and Physical Reactions is a direct and unambiguous effort to prevent physical harm that can be caused by content on the web.
For many, a flashing animation is a minor annoyance. For others, particularly the estimated 1 in 4,000 people with photosensitive epilepsy, it can trigger a life-threatening seizure. This guideline is our commitment to ensuring that a website never causes physical harm. It also covers other physical reactions, such as vertigo and nausea, caused by excessive motion.
Level A: The Essential Safety Threshold
This is a non-negotiable, foundational requirement. It is so critical that any failure to meet it can interfere with a user's ability to use the entire page.
Success Criterion 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.
The Level AAA criteria in this guideline take safety a step further, aiming to eliminate all potential risks and sources of physical discomfort related to motion.
Success Criterion 2.3.2 Three Flashes
Web pages do not contain anything that flashes more than three times in any one second period.
Motion animation triggered by interaction can be disabled, unless the animation is essential to the functionality or the information being conveyed.
Guideline 2.3 is a powerful reminder that accessibility is not just about convenience; it's about health and safety. By adhering to these criteria, we protect users from serious health risks and create a web that is predictable, comfortable, and stable for everyone. It shows a deep level of care and respect for the user's well-being.
Have you ever come across a website with flashing or animated content that made you uncomfortable? How can we better educate designers and developers on the health-related implications of these design choices? Share your thoughts!
Today's topic addresses one of the most critical aspects of web accessibility: user safety. The Guideline 2.3 Seizures and Physical Reactions is a direct and unambiguous effort to prevent physical harm that can be caused by content on the web.
For many, a flashing animation is a minor annoyance. For others, particularly the estimated 1 in 4,000 people with photosensitive epilepsy, it can trigger a life-threatening seizure. This guideline is our commitment to ensuring that a website never causes physical harm. It also covers other physical reactions, such as vertigo and nausea, caused by excessive motion.
Level A: The Essential Safety Threshold
This is a non-negotiable, foundational requirement. It is so critical that any failure to meet it can interfere with a user's ability to use the entire page.
Success Criterion 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.
- * The Core Problem: Rapidly flashing content can trigger photosensitive seizures. The brain's response to flickering light can cause electrical disturbances that lead to a seizure. This risk is severe and can occur with as little as a few flashes.
* The Three-Flash Rule: The primary rule is simple: nothing on the page should flash more than three times within a one-second window. This is a clear, quantitative boundary for content creators.
* The "Below Threshold" Exception: This is the key part of the Level A criterion. Content can flash more than three times if the flashes are dim enough to stay below a certain luminosity threshold. There are two specific thresholds based on the size and intensity of the flash, with a stricter threshold for red flashes, as they are known to be more problematic for triggering seizures.
* Why It Matters: This criterion is part of Conformance Requirement 5: Non-Interference. A single, non-compliant element can render the entire web page unusable and unsafe for a user.
The Level AAA criteria in this guideline take safety a step further, aiming to eliminate all potential risks and sources of physical discomfort related to motion.
Success Criterion 2.3.2 Three Flashes
Web pages do not contain anything that flashes more than three times in any one second period.
- * The Difference from Level A: This criterion completely removes the "below threshold" exception. It's a "zero tolerance" policy for any flashing content that exceeds the three-flash rule, regardless of its luminosity. While Level A is a solid safety measure, Level AAA eliminates even the most marginal of risks by removing the technical loophole.
* Why Is This Necessary?: This higher standard ensures maximum safety and removes the need for developers to perform complex luminance calculations. When in doubt, a complete absence of flashing content that exceeds the three-flash rule is the safest and most accessible path.
Motion animation triggered by interaction can be disabled, unless the animation is essential to the functionality or the information being conveyed.
- * Beyond Flashes: This criterion addresses another type of physical reaction: vestibular disorders. For users with these conditions, animations like smooth scrolling, parallax effects, zooming, or animated page transitions can cause severe dizziness, vertigo, and nausea.
* Giving Control to the User: The solution is to provide a mechanism for users to turn off or significantly reduce these animations. Many modern operating systems have a "Reduce Motion" setting. This criterion requires websites to respect that setting or provide their own control.
* Examples:
* A website with a dramatic parallax scrolling effect should have a "Disable Animation" option in the user settings or respect the user's OS-level preference.
* The smooth zooming in on a map is generally considered "essential" and would be an exception to this rule.
Guideline 2.3 is a powerful reminder that accessibility is not just about convenience; it's about health and safety. By adhering to these criteria, we protect users from serious health risks and create a web that is predictable, comfortable, and stable for everyone. It shows a deep level of care and respect for the user's well-being.
Have you ever come across a website with flashing or animated content that made you uncomfortable? How can we better educate designers and developers on the health-related implications of these design choices? Share your thoughts!