2. Operable

2.3 Seizures and Physical Reactions (Level AAA)

Success Criterion 2.3.2 Three Flashes

Level AAA

Web pages do not contain anything that flashes more than three times in any one second period.

Three Flashes Explained

Some people are so sensitive that it is not possible to completely prevent them from having seizures. However, by eliminating all flashing between three and 50 flashes per second everywhere on the screen, the chances of a person having a seizure are reduced.

SC 2.3.1 Three Flashes or Below Threshold (Level A) allows flashing if it is dim enough or is confined to a small enough area. SC 2.3.2 (Level AAA) does not allow flashing at all between three and 50 flashes per second, regardless of brightness or size.

Even a single flashing pixel violates this criterion. The intent is to guard against flashing areas that are larger than a single pixel; but, since an individual may require magnification or high contrast settings, the prohibition is against any flashing.

Try This: What Does Three Flashes Per Second Look Like? 

Warning: If you are sensitive to flashing, don’t do this activity.


To experience three flashes per second, open the MoodLight at the link above; choose two opposing colours that contrast well, like red and green; adjust the slider to “3 times per second” then press the “Turn On” button. Also, try flashing between 10 and 25 times per second — the range where the greatest sensitivity occurs — to experience what these frequencies look like.

Note: The script that controls the flash rate does not work well at higher rates, so it isn’t currently possible to experience the point at which the flashing becomes solid (at about 50 hz).

Suggested Reading:

Success Criterion 2.3.3 Animation from Interactions

WCAG 2.1

Level AAA

Motion animation triggered by interaction can be disabled, unless the animation is essential to the functionality or the information being conveyed.

Animation from Interactions Explained

While SC 2.2.2 Pause, Stop, Hide (Level A) addresses animation that is part of web content, SC 2.3.3 Animation from Interactions (Level AAA) addresses cases where movement occurs on the screen as a result of the user interacting with the content.

People with vestibular disorders, which affect eye movement and balance control, may become dizzy (vertigo) or experience nausea or headaches when encountering content of significant size moving across their visual field. Rapidly paging through an ebook mimics the effect. A similar effect is motion sickness caused by looking out a side window of a moving car. Effects can be quite severe, potentially causing vomiting and requiring bed rest to reduce the symptoms.

As a result, any user-initiated animation should be avoided, or users should be provided with a way to disable such animations.

This success criteria does not apply to essential user-initiated animation. Scrolling, for example, is essential. In this case, users are in control and can adjust their scrolling speed to avoid any motion-related symptoms.

One form of motion in web content that is known to initiate vestibular dysfunction is parallax scrolling. This occurs when, while scrolling, the foreground and background scroll at different rates. If there is a significant difference in the scrolling rate, vestibular symptoms can be brought on.

Try This: Animation and Motion Sickness

Warning: If you know you are sensitive to motion sickness be careful with this activity or avoid it altogether.

If you have not experienced motion sickness before, this activity may not affect you. For many, though, animations like the following, when stared at for a period of time, can make them feel queasy. For those with a significant vestibular disorder, it may not take long before they start to feel sick. Scan through the following animations and choose one of the larger ones, with motion across the whole image. Stare at the image for a minute or two. Stop if or when you begin to feel off.

Did you experience any reaction?

Page with motion GIFs (click on an image to view it on its own, or choose one below):

Suggested Reading:


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Introduction to Web Accessibility Copyright © 2019 by The Chang School, Toronto Metropolitan University is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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