WebsightWhat it asks for, in plain English
No part of the page may flash more than three times in any one-second period, unless the flashing area is small enough and dim enough to fall below the published thresholds for general and red flashes. A flash is a pair of opposite changes in brightness, so bright-dark-bright is one flash.
This covers everything: video, animated images, CSS and JavaScript effects, strobing adverts, lightning in a trailer, and rapid transitions in a games clip.
It is the one criterion where a failure can cause physical harm. A seizure can be triggered within seconds, so there is no 'the person can look away'.
This is an independent explanation. The binding wording is the W3C Recommendation, and the W3C's own Understanding 2.3.1 page has the full intent, techniques and definitions.
Who it affects
Someone with photosensitive epilepsy can have a seizure triggered by a few seconds of strobing in a promotional video.
A person with migraine gets an attack from a flickering banner.
Anyone with a sensory sensitivity finds flashing content painful and leaves the page.
How to check it
- List every video, animated image and animated effect on the site, including adverts and third-party embeds.
- Watch each one through, looking for rapid changes between light and dark or flashes of saturated red.
- For anything that looks close, run the video through the free Photosensitive Epilepsy Analysis Tool (PEAT) from the Trace Center, which measures it against the thresholds.
- Check scripted effects such as hover glitches, loading flickers and error shakes.
What Websight checks for 2.3.1
Websight has no automated check for this criterion. It lists it in the manual checklist that comes with every report, so it is not forgotten.
Only a person can check this, ideally with a flash analysis tool. A scanner cannot watch a video or an animation.
Practical tips
- Brief video editors and ad suppliers: no strobing, no rapid cuts between bright and dark frames.
- If a clip genuinely needs flashing, put a warning and a play button in front of it; never autoplay it.
- Keep flashing content, if any, to a small area of the screen and away from saturated red.