Flickering, Blinking, Movement, and Refresh

Accessibility Standards Checklist

Requirement

  • Until user agents allow users to control flickering, avoid causing the screen to flicker.
  • Until user agents allow users to control blinking, avoid causing content to blink (i.e., change presentation at a regular rate, such as turning on and off).
  • Until user agents allow users to freeze moving content, avoid movement in pages.
  • Until user agents provide the ability to stop the refresh, do not create periodically auto-refreshing pages.

Reason

Displays that flicker or flash can cause photosensitive epileptic seizures in susceptible individuals, particularly if the flash has a high intensity and is within the frequency range between 2 Hz and 55 Hz. This includes flashing text, turning graphics on and off, or repeatedly changing between different images on the screen. Screens that refresh to new content without warning can be confusing for visually impaired users.

Examples

Flickering

A flickering or flashing screen may cause seizures in users with photosensitive epilepsy. Seizures can be triggered by flickering or flashing in the 4 to 59 flashes per second (Hertz) range with a peak sensitivity at 20 flashes per second as well as quick changes from dark to light (like strobe lights).

Blinking

If blinking content is used, provide a mechanism for stopping the blinking. Do not use the BLINK and MARQUEE elements.

Movement

Use Style Sheets to create or show movement, but if style sheets are turned off the movement will not appear. Therefore, nothing conveying important information should be moving.

Refresh

To refresh a page, use either of these options:

  1. Configure the server to use the appropriate HTTP status code (301). Using HTTP headers is preferable because it reduces Internet traffic and download times and it may be applied to non-HTML documents.
  2. Replace the page that would be redirected with a static page containing a normal link to the new page.

One may use the refresh option when refreshing to a secure site or to show that the URL has changed.

Suggestions for testing this on your pages

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