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Clarification of duration of flashing or flickering (Success Criteria 2.3.1)

From: Phill Jenkins <pjenkins@us.ibm.com>
Date: Tue, 27 Oct 2009 22:42:10 -0500
To: w3c-wai-ig@w3.org
Cc: public-comments-wcag20@w3.org
Message-ID: <OFFFC7AFE6.86A4D01A-ON8625765D.000EB704-8625765D.0014575E@us.ibm.com>
Is the duration of the flashing or flickering (i.e., more than one second 
of time) a factor in determining accessibility compliance to WCAG 2.0 
        See Success Criteria 2.3.1 http://www.w3.org/TR/WCAG20/#seizure
        "... do not contain anything that flashes more than three times in 
any one second period..."

In my special case example, there is a slightly less than 1 second 
duration of 3 flashes in an Adobe Flash image at the top of a draft 
Shanghai EXPO 2010 page. 

WARNING - currently it flashes 3 times for slightly less than one second, 
then repeats several seconds later.
        see http://fr.expo2010.cn/indextest.htm

Assuming the developer adds a capability to stop the repeating Adobe Flash 
movie, is it still a problem because it flashes 3 times, regardless of the 
duration (e.g., 1 second, 10 seconds, etc.)?  Or does it need to flash 
more that 3 times a second of time for more that 1 second of duration?

The "Understanding Guidelines 2.3" further mentions: "likely to cause 
seizure when viewed even for a second or two."
        see http://www.w3.org/TR/UNDERSTANDING-WCAG20/seizure.html

metrics to consider: Frequency, Duration, and Size (the text only really 
address frequency and size, not duration)
        frequency:      (or interval) more than 3 flashes per second of 
time? (3 to 50 Hz) 
        duration:       more than one second duration of time? if it 
flashes more than 3, but for less than 1 second, is it a problem?
        size:           341 x 256 pixel block or larger - see 

Since my example only flashes 3 times and not more than 3, and it soon may 
change to not repeat - Guidelines 2.3 says "more than three", then I think 
I do not have a problem, but I'm not sure - hence the question to the 
list.  Your informed opinions welcomed.

I also believe the text and explanation can be simplified.  For example:
1. by addressing the three different metrics; frequency, duration, and 
2. by addressing the lower and upper limits of each.  For example, if the 
frequency is 40 times in a second, but only lasts 1 second or less, 
regardless of size it may be hardly noticeable and not be a problem. 
3. add some more common passing and failing examples.  The two existing 
ones are good. 

Phill Jenkins, 
Received on Wednesday, 28 October 2009 03:42:45 UTC

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