WCAG 2.3–Seizures and Physical Reactions

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

Success Criterion 2.3.1: Three Flashes or Below Threshold

A web page shouldn’t contain any content that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds.

Individuals who have photosensitive seizure disorders can have a seizure triggered by content that flashes at certain frequencies for more than a few flashes. People are even more sensitive to red flashing than to other colors, so a special test is provided for saturated red flashing. These guidelines are based on guidelines for the broadcasting industry as adapted for computer screens, where content is viewed from a closer distance (using a larger angle of vision).

Official Requirement

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. 

Note: Since any content that does not meet this success criterion can interfere with a user’s ability to use the whole page, all content on the Web page (whether it is used to meet other success criteria or not) must meet this success criterion.

How to Fix

• Don’t allow any part of the content to flash more than three times in a second.
• If the content must flash more often, make sure that it’s smaller than than  25% of 10 degrees of visual field. For web content, assuming a monitor size of 1024 x 768, a contiguous area of 21,824 sq pixels (any shape). This is equivalent to an area of about 341 x 256 pixels.

Source: https://www.boia.org/wcag2/cp/2.3.1

Success Criterion 2.3.2: Three Flashes

The purpose of this Success Criterion is to further reduce the chance of seizures. Seizures cannot be completely eliminated since some people are so sensitive. However, by eliminating all 3-per-second flashing over any area of the screen, the chances of a person having a seizure are further reduced than when just meeting the measures ordinarily used today in standards internationally, as we do at Level A. 

Whereas Success Criterion 2.3.1 allows flashing if it is dim enough or has a small enough area, Success Criterion 2.3.2 does not allow flashing greater than 3 per second, regardless of brightness or size. As a result, even a single flashing pixel would violate this criterion. The intent is to guard against flashing larger than a single pixel, but since an unknown amount of magnification or high contrast setting may be applied, the prohibition is against any flashing.

Benefits

• Individuals who have seizures when viewing flashing material will be able to view all of the material on a site without having a seizure and without having to miss the full experience of the content by being limited to text alternatives. This includes people with photosensitive epilepsy as well as other photosensitive seizure disorders.

Source: https://www.w3.org/WAI/WCAG21/Understanding/three-flashes.html

How We Test Video

The Photosensitive Epilepsy Analysis Tool (PEAT) is used to test videos that will be placed on UAMS websites. PEAT is a free, downloadable resource for developers to identify seizure risks in their web content and software. The evaluation used by PEAT is based on an analysis engine developed specifically for web and computer applications.