(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 peak sensitivity range appears to be 20 flashes per second. It has also been discovered that quick changes from dark to light (like strobe lights) can trigger an event. 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.

As a precaution, do not use the <BLINK> or MARQUEE constructs.

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