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. (Level A)


People who suffer from photosensitive seizure disorders, more specifically those who have photosensitive epilepsy induce seizures when a flash appear on the screen. The flash effect is observed significantly high in people who use screen magnifiers to read the content of the page and those who have problems such as tunnel vision.

What is Photosensitive epilepsy?

Photosensitive epilepsy is when seizures are triggered by flashing lights or contrasting light and dark patterns. Around 1 in 100 people has epilepsy and of these people, up to 5% have photosensitive epilepsy. Photosensitive epilepsy is more common in children and young people and is less commonly diagnosed after the age of 20.

What is Flashing content?

Switch back and forth between two visual states in a way that is meant to draw attention of the user is blinking. A pair of opposing changes in relative luminance that can cause seizures in some people if it is large enough and in the right frequency range is flash.

Effect of flashing content

Anything that flashes more than three times in a second cause seizures to the users. Also if the flashing area occupies large viewable portion on the screen, it may cause seizures. On a screen of 1024 x 768 resolution, any flashing content occupying a combined area more than 341 x 256 pixels fails this SC. 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.

Effect of Flashing content on Web

It is very less common having flashing content on websites. This kind of content is observed more on animated movies, video games and television programs. 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. Ensure that flicker that violates the flash thresholds is not caused by the content itself which can be taken care by designers and content authors.

Points to Remember

  • 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.
  • One transition from light to dark and dark to light or vice versa is considered to be one flash.
  • Blinking in general does not occur more than 3 times in a second, if anything that blinks more than three times in a second it is considered to be flash.
  • Providing a mechanism to switch-off flash on a web page is not a good practice as the users will not be able to trigger the off button before the flash occurs.
  • Providing warning of a flash content is even not a good practice as children are more likely to hit with photosensitive seizure disorders and they might not be consider warnings.
  • Avoid red flash as it has the longer wave length that stimulates cones in the retina.
  • The flash effect is observed significantly high in people who use screen magnifiers to read the content of the page and those who have problems such as tunnel vision.

Techniques for 2.3.1 Three flashes or Below Threshold