In plain language
Nothing may flash more than 3 times per second (seizure risk).
Who it affects
People with photosensitive epilepsy and others sensitive to rapid flashing.
How to check
Review videos, GIFs and effects for rapid flashing; use the PEAT tool for anything borderline. Worked example — A grocery delivery app's shopping cart: review third-party ad creatives or user-generated content embedded on the page: unscreened content with flashing above threshold fails.
Common misconception
Marketing teams sometimes assume flashing is only a concern for full-screen strobe effects. Even a smaller flashing region can trigger seizures if it covers enough of the visual field and flashes fast enough. Real example: on an email client's compose window, a loading animation flashes bright red and white more than three times per second while the compose window saves a draft, with no way to disable or reduce it.