The criteria for presenting small flashing objects and text on a web page varies depending upon the reference you use when it comes to defining what amount of flashing and moving is permitted on the web within the web accessibility laws. Of course, this changes depending upon which country’s laws are being followed as well.
The important point to consider is that you don’t let small flashing objects flash faster than 3 times per second. According to W3C.org:
The criterion is 25% of any 10 degree visual field, any single flashing event on a screen (there is no other flashing on a screen) that is smaller than a contiguous area of 21,824 sq pixels (any shape), would pass the General and Red Flash Thresholds.
That is an image the size of 341 x 246 pixels. The definition of a “general flash” or a “red flash” are defined below in the threshold definition by WCAG 2.0 Recommendation as:
a flash or rapidly changing image sequence is below the threshold (i.e., content passes) if any of the following are true:
- there are no more than three general flashes and / or no more than three red flashes within any one-second period; or
- the combined area of flashes occurring concurrently occupies no more than a total of .006 steradians within any 10 degree visual field on the screen (25% of any 10 degree visual field on the screen) at typical viewing distance
Squizlabs’ HTML_CodeSniffer describes this as:
The definition of small enough is given as “25% of any 10 degree visual field on the screen… at typical viewing distance”. This size, therefore, depends on the screen size, resolution, and viewing distance. The higher the resolution or screen size, the larger the permitted area.
Objects that flash and blink on the web page run the risk of initiating optically-induced seizures in visitors. According to WebAIM, no element should flash at a rate of 2 flashes per 55 cycles per second. Using their 508 checklist, one or more elements that flicker on a page that does not meet this criteria is considered to fail, and will put someone at risk who is sensitive of optically-induced seizures.
Three Flashes or Below Threshold: web pages do not contain anything that flashes more than three times in any one second period.
The intent is to reduce the chance of seizures since some people are sensitive to flashing on screens. This includes people with photosensitive epilepsy as well as photosensitive seizure disorders.
W3C goes on to further explain this is detail so that a writer could fully understand how critical it is to make sure accessibility standards are met. The other resources all show that optical seizures are the main reason why that this criteria is maintained. These sources differ in the timing of the flashing time limit but they agree about the results. So whatever source you choose consider your reader.
The web standards for moving and flashing text for web accessibility are found in Section 508 1194.22 (i), WCAG 1.0 A 7.1, and WCAG 2.0 A 2.3.1.