‘Three flashes or below threshold’ requires that no content flashes more than three times per second.


Flashing content on a website can cause difficulties for users with photosensitive seizure disorders such as epilepsy. Flashing content can cause these users to suffer a seizure.

How to Pass ‘Three Flashes or Below Threshold’

Don’t add anything to your website that flashes more than three times per second.


There’s one exception to this guideline based on the size of the flashing content, but I recommend you ignore it and just don’t let anything flash more than three times per second.

“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.”

Understating Success Criterion 2.3.1

If you can understand that you’re smarter than me!

‘Three Flashes or Below Threshold’ Tips

Remember, flashing is different to blinking (see Pause, Stop, Hide). Blinking can distract users but doesn’t cause seizures. 

If blinking content occurs three times per second, it is considered flashing content.

The exception is removed at Level AAA in Three Flashes.

See Also

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7 comments on “Three Flashes or Below Threshold (2.3.1 – Level A)

    Richard says:

    One thing that is important to check is that any video conforms to this, and that means checking anything that is added to your site. It is probably the reason for the technical exception, because a small video window may well pass. They announce on the news if there will be any flash photography in a clip for good reason, and exactly the same applies to the web, even some film trailers can fall foul of the guideline. Of course that can be difficult if for example you publish many videos.

    However, it is also important to put this in perspective. The age standardised estimate for prevalence of epilepsy is 0.75% (source here) and the estimate for prevalance of photosensitive epilepsy is 3% of those who have epilepsy (source http://www.epilepsy.org.uk/info/photosensitive-epilepsy), so only a small number of people are potentially affected.

    That isn’t to say that the guideline should be ignored, and photosensitive epilepsy is a serious condition.

    Luke McGrath says:

    Thanks for your perspective Richard, have you come across a plain English explanation of the exception? I’d love to give my readers a proper understanding of it, if it can help at all.

    Oisin Connolly says:

    I didn’t even know people are putting strobe effects on websites…. I’ve only ever encountered it in video, in which case any digital media artist who knows what their doing would put in a warning.

    Do you have any links to offending websites? Im curious to see what it would be like…I can’t imagine any website that flashes its users to death is going to have a good design or be very popular 😛

    Richard says:

    Sorry Luke, I forgot to reply to your question. I haven’t seen a plain English explanation unfortunately.

    Oisin, in all the many websites I have tested, I have only ever come across flashing in video. A common one is of course flash photography in news reporting, but sometimes I see something like a cinema trailer or similar where the picture or parts of it change so rapidly that it could be considered flashing. As you say, the important thing is a warning, and along with that comes the need to not auto start anything with flashing content.

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    Daniel Tonon says:

    I think the threshold might be for the sake of allowing something like a white blinking cursor on a dark background. It’s small enough that it is unlikely to cause a seizure.


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