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Flashing Content

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Summary

Section titled “Summary”

Flashing content can cause migraines and seizures. People can and have ended up in a hospital as a result of the effects of flashing content.

Flashing is any quick switch from high contrast to low contrast that happens more than 3 times a second but is slow enough to still be visible (3-50 flashes per second). Once common on websites, now it is more often seen in videos.

Given the serious consequences of using flashing content, it is strongly recommended that all flashing is avoided.

Overview

Section titled “Overview”

A Pokemon cartoon broadcast in Japan in 1997 contained repetitive flashing effects. As a result, 685 children were taken to hospital by ambulances and over 150 were hospitalized. Some of the children had blurred vision, headaches, dizziness, and nausea. Others experienced blindness, convulsions, seizures, and loss of consciousness. Two of the children remained hospitalized for over two weeks.

The Pokemon Shock incident is not an isolated event. Beyond WCAG: Losing Spoons Online describes one person’s painful reaction to the use of flashing and animation.

While color, area, and timing make it possible to use flashing in ways that will have less impact, it’s easy to avoid and not worth taking a chance.

Photosensitive seizures can be triggered by:

  • Flashing or flickering lights.
  • Rapidly changing or alternating images or blocks of color.
  • Patterns such as stripes and squares, even if they do not move.

There are only very narrow parameters for the safe use of flashing. The negative impacts of flashing content are so serious that it is strongly recommended not to use flashing.

Who is Helped

Section titled “Who is Helped”

People with photosensitive epilepsy, as well as other photosensitive seizure disorders, are helped when flashing and strobing effects are not used. In addition to seizures, they can induce vertigo (dizziness), nausea, vomiting, migraines, shaking, and other symptoms.

Guidelines

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Flashing is any quick switch from high contrast to low contrast that happens more than 3 times a second but is slow enough to still be visible (3-50 flashes per second). Some people are even more sensitive to red flashing than to other colors.

The first and best guideline is to avoid all flashing content.

There are only very narrow parameters for the safe use of flashing. These parameters are highly technical, but in simple terms:

  • The rate of flashing must be less than 3 times per second.
    • For example, an alert that chimes 3 times to get users’ attention might also have a graphic that flashes 3 times, once per second, as an additional indicator for people who are Deaf or hard of hearing.
  • The area that flashes must be small.
    • For example, cursors, icons, or other small areas of flashing content, such as a page load indicator, can flash without causing a problem.
  • The flashing colors should not switch from dark to light colors, especially when one color is a saturated red.
    • For example, a light yellow area that switches to a light orange can flash without causing a problem.

To state this more accurately, any content that flashes more than 3 times per second must meet one of these two requirements to be used safely:

  • The combined, contiguous area that is flashing simultaneously is less than 25% of 10 degrees of visual field at a typical viewing distance.
    • For a screen size of 1024 x 768 pixels, this means the area of flashing must be 341 x 256 pixels or less.
  • The area that flashes must not include a saturated red AND the darkest image’s relative luminance is above 0.80, or the darkest image’s relative luminance is below 0.80 and the maximum change in relative luminance between the darkest image and the brightest image is less than 10%.

These requirements are very specific and highly technical. The WCAG specifications for flashing include detailed descriptions for all of these parameters:

Make sure you check all content, including any videos. This is often overlooked, and flashing or strobing effects in videos are relatively common.

Whenever flashing absolutely cannot be avoided, a warning must be given. The warning must be seen before the flashing occurs so that steps can be taken to avoid any negative impact.

Example of a warning:

WARNING: This video contains flashing images. Do not view this video if you are affected in any way by flashing or strobing effects. The information in the video can be safely accessed in a separate video transcript.

How to Test

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Start by manually inspecting the page and any related content, such as videos. Are there any flashing or strobing effects that flash more than 3 times in 1 second?

For any flashing or strobing effects with more than 3 flashes per second, a tool must be used to ensure that the flashing does not violate flashing thresholds. The Photosensitive Epilepsy Analysis Tool (PEAT) can be used to meet this requirement. The link to the tool also includes a User Guide, which should be read before using the tool to ensure the correct installation and configuration of your system.

See Learn about PEAT (Ta11y) for more information on using this tool.

References

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