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From: Terence de Giere
Date: Mar 6, 2002 9:18PM
Subject: Re: blink and marquee elements
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Besides the accessibility issue with the blink and marquee elements,
one needs to consider the usability issue - blinking or moving text can
also distract a user from something more important on a page,
particularly if the blinking or moving text is just a superficial effect
to make the page look more lively. They can be annoying to users.
On my computer blink flashes at 1.5Hz in Netscape 4.78, which would make
it just acceptable under Section 508 rules. Marquee changes at a faster
rate, but it is a question whether it falls under the heading of causing
"the screen to flicker". A note in the 508 specification under comments
says "Paragraph (k) limits the flashing or blinking rate of screen
items.", while the rule itself seems confined to the whole screen.
From the 508 rules formulated by the Access Board:
"The Trace Center expressed concern that research supported a limit of 3
Hz, not 2 Hz as described in the NPRM. Trace suggested that the flash or
blink rate avoid any flickering between (but not including) 3 Hz and 55
Hz, which is the power frequency for Europe."
The Access Board added further - "This provision is necessary because
some individuals with photosensitive epilepsy can have a seizure
triggered by displays which flicker or flash, particularly if the flash
has a high intensity and is within certain frequency ranges. The 2 Hz
limit was chosen to be consistent with proposed revisions to the ADA
Accessibility Guidelines which, in turn, are being harmonized with the
International Code Council (ICC)/ANSI A117 standard, "Accessible and
Usable Buildings and Facilities", ICC/ANSI A117.1-1998 which references
a 2 Hz limit. The Board agrees that an upper limit is needed, since all
electrically powered equipment, even an incandescent light bulb, has a
"flicker" due to the alternating current line voltage frequency (60 Hz
in the U.S., 55 Hz in Europe). There does not appear to be any
significant incidence of photosensitive seizures being induced by the
line voltage frequency of ordinary lights. Therefore, the provision has
been changed to prohibit flash or blink frequencies between 2 Hz and 55 Hz."
The idea of the accessibility guidelines is to prevent the screen from
strobing. If the whole screen strobes that is one kind of effect. If
just a small portion of the screen is moving or blinking, that might be
another effect. The ratio of the size of the blinking/moving object to
entire the visual field could have some important bearing on the
application of this rule. A person subject to seizures from just any
moving or blinking object would have a hard time moving through the
physical world. Does anyone know of any studies that discuss the size of
a blinking or moving object on a computer screen in relation to the
effect of producing seizures?
As proprietary HTML constructs, blink and marquee also only work on
specific browsers, blink on Netscape up through version 4 and marquee on
Internet Explorer. They appear as static text on the other browsers I am
familiar with, and they are not even cross-browser between Netscape and
Internet Explorer.
Terence de Giere
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