Ensure user control of time-sensitive content changes

These checkpoints are all about movement. The idea is to discourage you from using features that may disorient or confuse people.

The checkpoints are:

• Don't make the screen flicker [Priority 1]
• Don't make content blink [Priority 2]
• Avoid movement such as animated gifs or scrolling text in pages [Priority 2]
• Don't create pages that automatically refresh at regular intervals [Priority 2]
• Don't use HTML to automatically redirect pages [Priority 2]

Why the checkpoints are important

It is difficult for anyone to concentrate on a flickering screen. Screen flicker can also trigger epileptic seizures as follows:

• Flickering or flashing in the 4 to 59 flashes per second (Hertz) range. The peak sensitivity is at 20 flashes per second
• Quick changes from dark to light (like strobe lights)

It is also difficult to read content that flashes on and off or to read the rest of the page when something else is flashing. Blinking is particularly distracting for users who find reading difficult or who can't concentrate because of noise, stress or a learning disorder. Some screen readers can't read blinking text. They may stick on the text and read it repeatedly, or they may freeze the entire system.

Text or images that move continuously are distracting and annoying and also cause the following problems:

• Some people with cognitive or visual disabilities can't read moving text quickly enough
• Screen readers can't read moving text at all
• Users with poor vision find it hard to focus on moving content
• People with physical disabilities might not be able to move quickly or accurately enough to interact with moving objects. For example, if links appear as a continually scrolling list, you must read the list and click a link before it disappears. Clicking on moving links can be difficult for users with limited dexterity

Auto-refreshing and auto-direct pages pages can be confusing because they take control of navigation away from users, who can quickly become disoriented and fail to grasp the structure of a site.

People read at different rates. It is disruptive and unsettling if the page refreshes before you have a chance to determine the content of the page, or before you finish reading it. This can make pages or content unusable for users who have limited reading ability or have difficulty concentrating because of noise, stress or a learning disorder.

Auto refresh and redirect also causes problems for screen readers that generally cannot cope with auto-refresh or automatic re-directs and may fail to show any content at all.