A web page shouldn't contain any content that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds.
Individuals who have photosensitive seizure disorders can have a seizure triggered by content that flashes at certain frequencies for more than a few flashes. People are even more sensitive to red flashing than to other colors, so a special test is provided for saturated red flashing. These guidelines are based on guidelines for the broadcasting industry as adapted for computer screens, where content is viewed from a closer distance (using a larger angle of vision).
Official Requirement:
2.3.1 Three Flashes or Below Threshold: Web pages do not contain anything that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds.
Note: Since any content that does not meet this success criterion can interfere with a user's ability to use the whole page, all content on the Web page (whether it is used to meet other success criteria or not) must meet this success criterion.
How to fix:
- Don't allow any part of the content to flash more than three times in a second.
- If the content must flash more often, make sure that it's smaller than than 25% of 10 degrees of visual field. For web content, assuming a monitor size of 1024 x 768, a contiguous area of 21,824 sq pixels (any shape). This is equivalent to an area of about 341 x 256 pixels.
