Accessibility Testing Procedure

Three Flashes or Below

Guideline

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.

Intent

The intent of this regulation is to allow users to access the full content of a site without inducing seizures due to photosensitivity.

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).

Flashing can be caused by the display, the computer rendering the image or by the content being rendered. The author has no control of the first two. They can be addressed by the design and speed of the display and computer. The intent of this criterion is to ensure that flicker that violates the flash thresholds is not caused by the content itself. For example, the content could contain a video clip or animated image of a series of strobe flashes, or close-ups of rapid-fire explosions.

This regulation replaces a much more restrictive criterion in WCAG 1.0 that did not allow any flashing (even of a single pixel) within a broad frequency range (3 to 50 Hz). The updated guideline (WCAG 2.0) is based on existing specifications in use in the UK and by others for television broadcast and has been adapted for computer display viewing. The 1024 x 768 screen is used as the reference screen resolution for the evaluation. The 341 x 256 pixel block represents a 10 degree viewport at a typical viewing distance. (The 10 degree field is taken from the original specifications and represents the central vision portion of the eye, where people are most susceptible to photo stimuli.)

Finding Applicable Components

• Identify any content elements that may blink, flash, scroll, or otherwise move.

Inspecting and Using Components

• Check that there are no more than three flashes during any 1-second period.
• If there are three flashes, check that the Light/Dark status at the end of the 1-second period is the same as at the start.

Section 508/WCAG 2.0 Failure Conditions

• Any instance of content flashing more than 3 times each second:
  • Fails 1194.31(j): No flickering Interface components.
  • 2.3.1 Three Flashes or Below Threshold.

Reporting Test Results

• Any failure in Section 508 Failure Conditions results in: Fails Baseline Requirement #10.
• Any instance where content flashes less than 3 times each second or does not flash at all then: Passes Baseline Requirement #10.

For sharing test results between Agencies, the results of Section 508 Conditions and Reporting Test Results must be reported.