Section 508 - Standard (j)
Pages shall be designed to avoid causing the screen
to flicker with a frequency greater than 2 Hz and lower than 55 Hz.
Explanation - Standard
(j) - Screen Flicker
Elements that flicker include:
- animated gifs
- text that has been formatted to blink using the HTML <blink>
tag
- text that has been formatted to scroll using the HTML <marguee>
tag
- Macromedia Flash animations
There are undoubtably more elements that could be added to this list.
There are several reasons to avoid flickering screen elements.
- Elements that flicker between the rate of 2 Hz and 55 Hz may cause
seizures in individuals that have photosensitive epilepsy.
- Elements that move may be difficult to view for individuals that use
screen magnifying techniques.
- <blink> and <marguee> are not valid HTML coding. They
are also an annoyance to many users.
Techniques for Elements that
May Flicker
There are elements that designers include in web pages that can and should
be eliminated. <blink> and <marquee> are both tags that are
no longer part of the HTML standard. In combination with the fact that
these tags produce results that are an annoyance to individuals with full
use of their sight and a possible hindrance to users that are blind or
have partial vision, they should be avoided.
Animations may have a real purpose within a web page. Without question
an animation that displays molecules bonding has a definitive purpose.
On the other hand, there are animations that exist for no reason other
than decoration. If animations are a necessary component of a web page,
avoid or limit flickering. If the element is necessary, provide a method
for users to stop the animation.
Summary
There isn't an easy method for you to use to measure the flicker rate
of a flickering element. Thus, the best solution is to limit use of such
elements.
Other Considerations
Macromedia Flash is one product that can produce practical animated page
elements. Flash MX has functions that allow designers to include accessibility
in their design. If you are a Flash user, visit the Macromedia Flash web
site to learn more about accessibility.
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Validation Technique(s)
The testing measures that follow can be used to evaluate compliancy of
your web page(s) with the concepts presented for Standard (j).
- View your web page in a browser. Observe for elements that flicker.
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 complete
a self-assessment on screen flickering
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