Amongst the Web Content Accessibility Guidelines 2.0 (WCAG 2.0) is guideline 2.3 that says 'Do not design content in a way that is known to cause seizures'. This relates to fast flashing images that can cause epileptic fits. Obviously web site owners should meet this criteria and avoid causing any of their visitors medical harm. The advisory guidance with the guideline is to avoid anything that flashes more than three times a second. There have been few instances of web content that falls into this category. The most likely cases are videos with a lot of flash photography, hence the warnings on the television news items.
However, I am convinced that there is a much wider problem with flashing and moving items on a web page. They are OK if they are part of the main content of the page, but they are a major distraction if they are a peripheral part of the page. The most obvious examples are adverts in the header or down the right hand side of a page. The continual movement and flashing makes it much more difficult to read the main content, which is why the visitor has come to the page.
Personally I will give up and go to another site if at all possible. What is certain is that I will not follow the link in the advert. I am writing this article now because I have just been reading an article on a site that I read regularly; it is has great content, is well laid out and is careful to be accessible (helped by an occasional friendly push from me). Unfortunately this week it had a fast moving animated gif (not quite in the epileptic range) that made it very difficult for me to read the articles. I wanted to read the article so I found a small non-changing window on my desktop and put it on top of the gif, not an ideal solution but it worked.
The moral of this story is for web site owners to be very cautious about peripheral animation. If it is really considered to be necessary it should go through its cycle once (to draw the visitors attention) and then stop. It could also have a replay button so an interested visitor could see it again.
Web sites that do not take note of this will, at best, have visitors who do not concentrate fully on the intended content and, at worst, will have visitors who move off the site without having read anything.