1.3 Avoid causing the screen to flicker
Contents
WAI checkpoint 7.1
Full WAI text: "Until user agents allow users to control flickering, avoid causing the screen to flicker."
A user agent is a piece of software for accessing Web content. User agents could be desktop graphical browsers, text browsers, voice browsers, mobile phones, multimedia players, plug-ins, and some software assistive technologies used in conjunction with browsers such as screen readers, screen magnifiers, and voice recognition software.
It is possible to cause the screen to flicker by inserting commands in the underlying HTML of a web page, usually to create a visual effect.
Not all user agents allow the user to control the rate at which the screen flickers. Until such time as they do, you must not create flickering effects.
Rationale
It is difficult to concentrate on a flickering screen. Screen flicker can also trigger epileptic seizures. People with photosensitive epilepsy can have seizures triggered by flickering or flashing in the 2 to 60 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).
Directions and Techniques
Ensure that screen flicker is within a safe range
Keep flickering to less than 2 flashes per second (Hertz) and do not change quickly from from dark to light.
Provide a mechanism or control for freezing flicker or movement
If you create flicker effects using Java applets or other techniques, you should provide an obvious control which allows the user to freeze movement or disable the flicker effect.
How you could check for this:
There are no specific test methods recommended for this guideline.
Guidelines for Web Accessibility - Priority 1- 1.1 Provide a text equivalent for every non-text element
- 1.2 Ensure that information does not rely on colour perception
- 1.3 Avoid causing the screen to flicker
- 1.4 Provide an auditory description of the visual information in multimedia presentations
- 1.5 For multimedia, ensure that timing of alternative descriptions is synchronised with the presentation
- 1.6 Use the clearest and simplest language appropriate
- 1.7 Identify language changes in text
- 1.8 Ensure that equivalents for dynamic content are updated when the dynamic content changes
- 1.9 For data tables, identify row and column headers
- 1.10 For complex data tables, use mark-up to associate data cells and header cells
- 1.11 Add titles to frames
- 1.12 Ensure that documents can be read without style sheets
- 1.13 Provide text links to emulate server-side image maps
- 1.14 Use client-side image maps rather than server-side image maps where possible
- 1.15 Ensure that scripts and applets that provide the only source of important functionality are directly accessible or compatib
- 1.16 Ensure that pages are usable without support for scripts, applets, or other programmatic objects
- 1.17 If you cannot make a page accessible, provide an equivalent accessible page
