Screen Animations

Blinking page elements, some animated features, and some multimedia features can trigger a seizure in people with photosensitive epilepsy, a condition in approximately five percent of people with epilepsy where exposure to flashing lights at certain intensities can trigger seizures. Triggers can include flickering television screens, certain video games, computer monitors, alternating patterns of different colors, strobe lights, sunlight flickering through trees, and other changes in light patterns.

Blinking page elements and scrolling text can be problematic for screen reader users, because their software is unable to read moving text. Some people with cognitive or visual disabilities are unable to read moving text quickly enough, if at all. Movement can also cause such a distraction that the rest of the page becomes unreadable.

REQUIRED:

Pages shall be designed to avoid causing the screen to flicker with a frequency greater than 2 Hz and lower than 55 Hz. [Section 508, Part 1194.22, Paragraph (j)]

Until user agents allow users to control flickering, avoid causing the screen to flicker. [W3C WCAG 1.0, Checkpoint 7.1 Priority 1]

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). [W3C WCAG 1.0, Checkpoint 7.2 Priority 2]

Until user agents allow users to freeze moving content, avoid movement in pages. [W3C WCAG 1.0, Checkpoint 7.3 Priority 2]

Avoid deprecated features of W3C technologies. [W3C WCAG 1.0, Checkpoint 11.2 Priority 2]

Web developers should avoid screen animations and flickering at any rate, if at all possible. Do not use scrolling text. Be mindful about the special effects you use in rich media, for example, Flash movies or video clips. Ensure that these elements conform to this checkpoint.

EXAMPLE:

Don't use the <BLINK> or <MARQUEE> elements. In addition to causing screen animations that the end user cannot control, they are not recognized as official HTML 4.01 elements.

In order to minimize the probability of inducing an epileptic seizure, screen elements that flicker or change should do so at a frequency of less than twice per second (2 Hz) or greater than 55 times per second (55 Hz).

Do not animate quick changes from dark to light that simulate strobe lights.

If screen animations are used, provide user control over the elements and ensure that these objects or pages may be paused or stopped.