2.2 Avoid causing content to blink
Contents
WAI checkpoint 7.2
Full WAI text: "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)."
User agents are software which people use to access Web content. These include graphical browsers, text-only browsers, voice browsers, mobile phones, multimedia players, plug-ins and assistive technologies such as screen readers, screen magnifiers and voice recognition software.
It is possible to cause content to blink or flash by inserting commands to do this into the web page. This is usually done to create a novel effect, but should be avoided at present, since not all user agents allow the user to control the rate of blinking or to disable it.
Rationale
Flickering may induce epileptic seizures in users if it occurs regularly at a particular rate.
It is also difficult to read content which is flashing on and off or to read content when something else on the page is flashing. Blinking is particularly distracting for users who have limited reading ability or difficulty concentrating because of noise, stress or a learning disorder.
Directions and Techniques
Ensure that screen flicker is within a safe range
Keep blinking to less than 2 flashes per second (Hertz) and do not change quickly from from dark to light.
Provide a mechanism or control for controlling or disabling blinking
If you create blinking 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 2- 2.1 Ensure that images have sufficient contrast for people with colour deficient vision
- 2.2 Avoid causing content to blink
- 2.3 Avoid movement in content
- 2.4 Ensure that dynamic content is accessible or provide an alternative presentation or page
- 2.5 Use style sheets to control layout and presentation
- 2.6 Use relative rather than absolute units
- 2.7 Use header elements to convey structure
- 2.8 Break up large blocks of information where appropriate
- 2.9 Ensure that information laid out using tables make sense when linearised, or provide an alternative equivalent
- 2.10 Do not use structural mark-up to format information laid out using tables
- 2.11 Describe the purpose of frames and how they relate to each other if it is not obvious by their titles alone
- 2.12 Clearly identify the target of each link
- 2.13 Provide information about pages and sites in metadata
- 2.14 Provide information about the general layout of a site
- 2.15 Use navigation mechanisms consistently
- 2.16 Associate labels explicitly with their controls
- 2.17 Properly position the labels of form controls
- 2.18 Ensure that user interfaces are device-independent
- 2.19 Use logical event handlers in scripts
- 2.20 Use input device-independent event handlers in scripts and applets
- 2.21 Ensure that scripts and applets are accessible
- 2.22 When an appropriate markup language exists, use markup rather than images to convey information
- 2.23 Create documents that validate to published formal grammars
- 2.24 Mark up lists and list items using the proper HTML tags
- 2.25 Use quotation mark-up for quotations, but not for formatting
- 2.26 Where possible, use appropriate W3C technologies of the latest supported versions
- 2.27 Avoid deprecated features of W3C technologies
- 2.28 Do not periodically auto-refresh pages
- 2.29 Do not use mark-up to redirect pages automatically
- 2.30 Do not generate pop-ups or other windows and do not change the current window without informing the user
