Good examples of Guideline 2.3 Seizures

We will not cause the screen to flash in a manner which might induce a seizure. This is due to Photosensitive Epilepsy, which can be fatal.

There is some guidance for calculating the "safe area" a flashing object occupy on a page

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.

For general software or web content, using a 341 x 256 pixel rectangle anywhere on the displayed screen area when the content is viewed at 1024 x 768 pixels will provide a good estimate of a 10 degree visual field for standard screen sizes and viewing distances (e.g., 15-17 inch screen at 22-26 inches). (Higher resolutions displays showing the same rendering of the content yield smaller and safer images so it is lower resolutions that are used to define the thresholds.)