Three Flashes
Nothing on the page may flash more than three times in any one second. The Level A rule lets flashing through when it is dim enough or small enough. This one takes both allowances away, at any brightness and any size, and W3C means that literally. A single flashing pixel breaks it, because a reader may be magnifying the page and nobody can know by how much.
Why it matters
The Level A thresholds are the measures used in standards internationally, and W3C's stated reason for going further at AAA is that removing all three-per-second flashing reduces the chance of a seizure further still. That is the stake here. Not an annoyance, a seizure. It is also why this is the one operable rule with no user control offered anywhere in it. W3C explains that plainly. Blinking is a distraction you can allow briefly as long as it can be stopped. Flashing would trigger the seizure faster than anybody could reach the off switch.
Who this rule protects
People with photosensitive epilepsy and other photosensitive seizure disorders are who this protects.
How to check it yourself
- Review video and animated graphics for anything that flashes more than three times a second.
- Check loading indicators and transitions for a strobe effect.
- Count first. If nothing flashes more than three times in a second, the content passes and no tool is needed.
- Where there is real doubt, run a photosensitivity analysis. W3C names PEAT and Harding FPA.
Failures we see most often
- Stock footage cuts rapidly between bright and dark frames.
- An animated GIF carries a strobing highlight that repeats too fast.
- A small strobe gets defended on the grounds that it is too small to matter, when this criterion has no size allowance at all.
Who this one is for
Read from this rule's own note above, so the grouping and the note cannot disagree.
- Seizures and vestibular disorderspeople for whom motion or flashing causes harm
How to fix it
- Re-edit or replace footage that flashes too fast. W3C's own example is a lightning sequence cut back to three flashes a second.
- Slow any pulsing indicator well below three flashes per second.
- Reduce the contrast of anything that has to blink at all.
Step-by-step fix guides (1)
Passes vs. fails
Passes
The same sequence gets cut, and the rest of the edit never crosses three flashes in a second.
Fails
A hero video runs a strobe sequence small enough to clear the Level A area threshold, and it still fails here.
In audits and lawsuits
This is Level AAA and, unlike most of AAA, usually free, because almost nothing on a website needs to flash three times a second. There is a shortcut worth knowing before anybody buys tooling. W3C states that no analysis tool is needed at all where the flashing is three times a second or fewer, because that content passes automatically. Where there is genuine doubt, W3C names two analyzers itself. PEAT from the Trace Center, and Harding FPA. One asymmetry to note as well. The Level A version of this rule carries a non-interference obligation, so it reaches every piece of content on the page whether or not that content is used to meet anything else. This AAA version carries no such clause.