Chapter 15: Flashing Content

Any flashing content on the computer screen for more than 3 times in one second can cause seizures. Seizures can be caused due to photo sensitivity.

The fundamental concepts to keep in mind to test this requirement are,

  1. Is the flashing happens more than 3 times in one second;
  2. Is the flashing content large enough.

Is the flashing happens more than 3 times in one second?

The opposing changes in relative luminance either from dark to light or light to dark is considered as one flash. If these flashing is observed more than 3 times in one second, this is a violation. Red flash is more sensitive to seizures and authors need to be extra careful in using them.

Note: Warning for the user or a mechanism for stopping the flash cannot be considered as exceptions for this requirement.

Is the flashing content large enough?

Any flashing content that is covers larger screen area than 341 x 256 pixel block on a standard 1024 x 768 pixel screen is a problem for the users. This covers a larger portion i.e. 10 degrees area on the screen as the users of computer work from a closer distance. This area can also be a combined portion of different blocks within 10 degrees of viewable area.

Exception:

Sometimes flashing can be caused due to the display or the computer rendering the image. Content authors will not have control over these types of flash.

Note: Blinking usually do not happen more than 3 times in one second. IF it happens then it is considered as flashing.

Validation for flashing content

  1. Open the page on a browser.
  2. Check if the page contains any flashing content.
  3. If no flashing content is available, this success criteria is considered as not applicable.
  4. If flashing content is observed, check for
    1. Is the flashing happens more than 3 times in one second?
    2. Is the flashing content large enough?
  5. If any one of the requirements in check 4 is observed, this success criteria is considered as failed else passed.

WCAG Success Criteria

2.3.1 3 flashes or below threshold