2. Operable

2.3 Seizures and Physical Reactions (Level A)

Guideline 2.3 Seizures and Physical Reactions

Do not design content in a way that is known to cause seizures or physical reactions.

What do I need to know so that content does not cause seizures?

People who have epilepsy can have seizures when exposed to flashing or flickering lights. There are three causes of flickering lights on computer screens:

  1. Flashes can be caused by the display.
  2. Flashes can be caused by the computer and how it renders images and other content.
  3. Flashes can be caused by the content itself.

Although web authors have no control over the first two, they can ensure that flicker is not caused by the content, such as a movie of strobe flashes or an animation of rapid-fire explosions.

To conform at Level AA, WCAG 2.1 describes how to determine safe values for flashing content. To conform at Level AAA, a page must avoid all flashing content.

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.

Three Flashes or Below Threshold Explained

People with a form of epilepsy called photosensitive seizure disorder have seizures when exposed to flashing or flickering lights. Many colours can cause these seizures, but flashing red lights are known to trigger seizures more readily than other colours, and the larger the area flashing, the greater the possibility of inducing a seizure.

To meet this requirement, web authors must ensure that flickering is not caused by the content itself, such as a movie of strobe flashes or an animation with rapidly switching colours.

SC 2.3.1 allows content to flash if it is dim enough and confined to a small area. W3C publishes formulas for determining safe values for “general flash” and “red flash” thresholds. But even these so-called safe values can trigger seizures. Research suggests the greatest sensitivity to flashing is between 10 hz and 25 hz. The next section, SC 2.3.2, is more stringent.

Try This: Flashing Screens

Humans can only detect flashing (i.e., the flicker rate) up to about 50 hz (or 50 flashes per second), after which a screen or content appears continuous. You may recall seeing computer or television screens flickering or the rolling bar that cycles across the screen in cases where a monitor is being viewed in a video or TV program. The cause is the difference in the flash rate of the monitor and the flash rate of the camera recording the monitor.

Even though you cannot see it due to the flash rate being faster than the eye can detect, all screens do flicker.

See the flash of a video recorded monitor:

Video: How to make screens look good on camera by Kyle Lawrence

Movie and television producers are now quite aware of flashing screens. Technology is used to prevent it from occurring, which is why you may need to go back to fairly old movies or television programs to see it happening.

Suggested Reading:

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Introduction to Web Accessibility by Ryerson University, The Chang School is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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