Understanding 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.

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. See Conformance Requirement 5: Non-Interference.

Intent of Success Criterion 2.3.1: Three Flashes or Below Threshold

The intent of this Success Criterion 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 Success Criterion 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). This Success Criterion 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.)

The combined area of flashes occurring concurrently and contiguously means the total area that is actually flashing at the same time. It is calculated by adding up the contiguous area that is flashing simultaneously within any 10 degree angle of view.

The terms "blinking" and "flashing" can sometimes refer to the same content.

  • "Blinking" refers to content that causes a distraction problem. Blinking can be allowed for a short time as long as it stops (or can be stopped)
  • "Flashing" refers to content that can trigger a seizure (if it is more than 3 per second and large and bright enough). This cannot be allowed even for a second or it could cause a seizure. And turning the flash off is also not an option since the seizure could occur faster than most users could turn it off.
  • Blinking usually does not occur at speeds of 3 per second or more, but it can. If blinking occurs faster than 3 per second, it would also be considered a flash.

Benefits of Success Criterion 2.3.1: Three Flashes or Below Threshold

Examples of Success Criterion 2.3.1: Three Flashes or Below Threshold

Resources Success Criterion 2.3.1: Three Flashes or Below Threshold

Techniques for Success Criterion 2.3.1: Three Flashes or Below Threshold

Sufficient Techniques

  1. Ensuring that no component of the content flashes more than three times in any one second period
  2. Keeping the flashing area small enough
  3. Using a tool to ensure that content does not violate the general flash threshold or red flash threshold

Advisory Techniques

  • Reducing contrast for any flashing content (future link)
  • Avoiding fully saturated reds for any flashing content (future link)
  • Reducing the number of flashes even if they do not violate thresholds (future link)
  • Providing a mechanism to suppress any flashing content before it begins (future link)
  • Slowing down live material to avoid rapid flashes (as in flashbulbs) (future link)
  • Freezing the image momentarily if 3 flashes within one second are detected (future link)
  • Dropping the contrast ratio if 3 flashes within one second are detected (future link)
  • Allowing users to set a custom flash rate limit (future link)

Failures