Web checklist

Checkpoint 2.3a: Flashing Content or Below Threshold

Web pages do not contain anything that flashes more than two times in any one second period, or the flash is below the general flash and red flash thresholds.


Rationale

The purpose of this checkpoint is to enable users to access the full content of a Web site without inducing seizures due to photosensitivity.

For people with photosensitive seizure disorders, Web content that flashes at certain frequencies for more than a few flashes can trigger a seizure. Because people are more sensitive to red flashing than to other colors, you must perform a special test for saturated red flashing. These guidelines are based on the broadcasting industry guidelines, but they have been adapted for computer monitors, which allow content to be viewed from a short distance (using a larger angle of vision).

Required development and unit test techniques


To comply with this checkpoint, you must meet all of the following techniques.

These techniques are defined in WCAG 2.0 Level A Success Criterion 2.3.1 and Section 508 standards 1194.22(j) (both links reside outside of ibm.com).

  1. No flashes between two and 55 times per second: Ensure that no component of the content blinks or flashes at a frequency between two times per second and 55 times per second.
  2. Small flash area: Keep the flashing area small enough.

General examples

  1. No flashes between two and 55 times per second: Ensure that no component of the content blinks or flashes at a frequency between two times per second and 55 times per second.

     

    To comply with this technique, you must implement all of the following examples.

    General example 1

    Ensure that content does not violate the general flash threshold or red flash. Displays that flicker or flash can cause photosensitive epileptic seizures in susceptible individuals, particularly if the flash has a high intensity and is within the frequency range between 2 to 55 times per second. This includes flashing text, graphics that turn on and off, or images that change repeatedly.

    For example, an animation of a thunderstorm shows six strikes of lightning. The strikes are so fast and large that the general flash threshold is violated when tested with a flash analysis tool. To ensure that the animation does not violate the general flash threshold, the animation is modified to create a short pause after each pair of lightning strikes.

  2. Small flash area: Keep the flashing area small enough.

    To comply with this technique, you must implement the following example.

    General example 2

    Keep the flashing area small enough by following these steps:

    1. To convert viewing distance to rectangle size, multiply the viewing distance by 0.1745 (10 * Pi / 180) to get the width of the rectangle. Multiply the viewing distance by 0.1309 (7.5 * Pi / 180) to get the height of the rectangle. This calculation can be done in inches, or millimeters, or any other unit of length.

    2. Determine the size of a 10 degree angle of view in pixels. Multiply the width and height of the rectangle from step 1 by the resolution of the screen, in pixels per unit length, to get the horizontal and vertical size of the rectangle in pixels.

    3. Multiply the width of the rectangle by the height and divide by 4.

      For example, an author creates an animation that will be displayed on a screen in the entrance lounge at a company. Using the size and resolution of the display and the closest distance that a person can stand when viewing the display, they calculate the size of 25 percent of the 10 degree of central vision in pixels (using the formula above). This would be the small safe area. They then are careful to never flash any area larger than the small safe area.

    Required unit tests for general development technique 1

    Manually perform the following unit tests.

    1. Identify any flashing content and verify that only one area of the screen is flashing at any time and the flashing content would fit into a contiguous container whose area is less than the small safe area.
    2. Keep the flashing area small.

    Additional test hint for flashing content

    When testing this requirement:

    1. Count the number of flashes in one second. Verify the number is two or fewer.
    2. If the flashing continues for several seconds, you may find it easier to count the number of flashes in 10 seconds and then divide by 10. Verify the resulting number is 2 or less.
    3. Check that the Light/Dark status at the end of the one-second period is the same as at the beginning.

Recommended development techniques

Although you do not have to implement the recommended techniques in order to comply with this checkpoint, you should review them because they can improve the accessibility and usability of the application.

Refer to the WCAG 2.0 Additional Techniques (Advisory) for Success Criterion 2.3.1 (link resides outside of ibm.com) for a list of techniques and examples.


©2013 IBM Corporation

Last updated January 1, 2013.

W3C Recommendation 11 December 2008: http://www.w3.org/TR/WCAG20/ (link resides outside of ibm.com)
Copyright 1994-2009 W3C (Massachusetts Institute of Technology, European Research Consortium for Informatics and Mathematics, Keio University), All Rights Reserved.