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

General comments

Some individuals with photosensitive epilepsy can have a seizure triggered by displays which flicker or flash, particularly if the flash has a high intensity and is within certain frequency ranges.  Seizures can be triggered by flickering or flashing in the 2 to 59 flashes per second (Hertz) range.  Developers should avoid using strobe light effects - abrupt changes from light to dark - or using flashes in any color range at a rate of 2 to 59 flashes per second.  Flashing or flickering elements are usually added through technologies such as animated gif's, Java applets, or third-party plug-ins or applications.  The <BLINK> tag should not be used.

Avoid causing the screen or its content to flicker or blink and avoid movement in pages.  When a page includes moving content, provide a mechanism within a script or applet to allow users to freeze motion or updates.  Using style sheets with scripting to create movement allows users to turn off or override the effect more easily.  Also, unless there is an ability for user to stop the refresh, do not create periodically auto-refreshing pages.

HTML Techniques for WCAG 1.0 /Screen Flicker