(j) Pages shall be designed to avoid causing the screen to flicker with a frequency greater than 2 Hz and lower than 55 Hz. 

Why is this provision necessary?
This provision is necessary because some individuals with photosensitive epilepsy can have a seizure triggered by displays that flicker, flash, or blink, particularly if the flash has a high intensity and is within certain frequency ranges. The 2 Hz limit was chosen to be consistent with proposed revisions to the ADA Accessibility Guidelines which, in turn, are being harmonized with the International Code Council (ICC)/ANSI A117 standard, "Accessible and Usable Buildings and Facilities", ICC/ANSI A117.1-1998 which references a 2 Hz limit. An upper limit was identified at 55 Hz.

How can flashing or flickering elements be identified?
Flashing or flickering elements are usually added through technologies such as animated gif's, Java applets, or third-party plug-ins or applications. Java applets and third party plug-ins can be identified by the presence of <APPLET> or <OBJECT> tags. Animated gif's are images that download in a single file (like ordinary image files), but have content that changes over short periods of time. Like other images, however, they are usually incorporated through the use of the <IMG> tag.

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11.  Blinking and Flicker

Relevance to Accessibility
This requirement is necessary because some individuals with photosensitive epilepsy can have a seizure triggered by displays that flicker or flash, particularly if the flash has a high intensity and is within certain frequency ranges. The 2 Hz limit was chosen to be consistent with proposed revisions to the ADA Accessibility Guidelines which, in turn, are being harmonized with the International Code Council (ICC)/ANSI A117 standard, "Accessible and Usable Buildings and Facilities", ICC/ANSI A117.1-1998 which references a 2 Hz limit. An upper limit was identified at 55 Hz.
Flashing or flickering elements are usually added through technologies such as animated gif's, Java applets, or third-party plug-ins or applications.

 
508 Software (Subpart B)(1194.21)
(k)
Software shall not use flashing or blinking text, objects, or other elements having a flash or blink frequency greater than 2 Hz and lower than 55 Hz.
508 Web (Subpart B)(1194.22)
(j)
Pages shall be designed to avoid causing the screen to flicker with a frequency greater than 2 Hz and lower than 55 Hz.
508 Functional Performance (Subpart C)
None or In Progress of Mapping
508 Information, Documentation, and Support (Subpart D)
None or In Progress of Mapping

 
Comparison

Comment:
We have explicitly chosen * not * to include this requirement for the user interface. We discussed whether our content requirements should be extended to the user interface in general, and decided against this.

UAAG 1.0 Requirements & Priority

 
3.3   Toggle animated / blinking text***
  1. Allow configuration to render animated or blinking text as motionless, unblinking text.
  2. In this configuration, the user must still have access to the same text content, but the user agent may render it in a separate viewport (e.g., for large amounts of streaming text).
Who benefits: Flashing content may trigger seizures in people with photosensitive epilepsy, or may make a Web page too distracting to be usable by someone with a cognitive disability. Blinking text can affect screen reader users, since screen readers (in conjunction with speech synthesizers or braille displays) may re-render the text every time it blinks.
[Priority 1]

 
@@
3.4 Toggle scripts
 
  1. Allow configuration not to execute any executable content (e.g., scripts and applets).
  2. In this configuration, provide an option to alert the user when executable content is available (but has not been executed).
  3. The user agent is only required to alert the user to the presence of more than zero scripts or applets (i.e., per-element alerts are not required).
Who benefits: Control of executable content is particularly important as it can cause the screen to flicker, since people with photosensitive epilepsy can have seizures triggered by flickering or flashing, particularly in the 4 to 59 flashes per second (Hertz) range. Peak sensitivity to flickering or flashing occurs at 20 Hertz.
[Priority 1]