This translation has not been editorially reviewed yet. The German version is authoritative. Deutsche Fassung

Accessibility

How often may a website flash per second?

A website must not flash more than three times in any given second. This limit from WCAG criterion 2.3.1 protects people with photosensitive epilepsy: rapidly flashing areas can trigger a seizure in them — with a risk of falling and injury. This mainly affects videos, animated ad banners, and effect animations with hard light-dark changes over a larger area.

Three flashes per second as the safety limit

In photosensitive epilepsy, the brain responds to rapid changes in brightness or colour with uncontrolled discharges. This isn't triggered by every animation, but by hard flashing: large-area, high-contrast changes in rapid succession, especially with intense red. WCAG draws the line at three flashes per second. In everyday web use, the danger rarely comes from the design itself — it's more often found in embedded videos, sweepstake animations, or third-party ad banners. Anyone publishing videos should review the material before release; strobe scenes should carry a warning beforehand, or better still, a defused version.

How the audit service handles flashing content

A reliable flash analysis would have to examine every video frame by frame for jumps in brightness, the area affected, and the proportion of red. A superficial quick test that then reports "passed" would be irresponsible for a health risk of this kind. The audit service therefore doesn't make an automatic judgment here: the report openly marks the criterion as a point for manual review. That's part of our principle of listing every WCAG criterion individually — passed, violated, needs manual review, or not automatable. An empty check counts as a failure, never as a free pass.

Examples

Blinking indication

Not met

.neu { animation: blink 0.2s infinite; }  /* 5 Wechsel je Sekunde */

Passes

.neu { animation: blink 1s 3; }  /* 1 Wechsel je Sekunde, endet nach 3 Sekunden */

More than three flashes per second can trigger a seizure. The safest: no blinking.

Review every video for strobe and flash effects before publishing it, especially event and product films.
Avoid ad banners and animations with rapid light-dark changes or intense red flashes.
Put a clear warning before unavoidable flash scenes and offer a defused version.
Ask about purchased ad slots what moving content is served there — third-party content still runs on your page.

Deutsche Fassung dieser Seite

FAQ

What is photosensitive epilepsy?

Photosensitive epilepsy is a form of epilepsy in which rapid changes in light and contrast can trigger seizures — such as strobe light, flickering screens, or flashing videos. A seizure can last from seconds to minutes and carries a risk of falling and injury. That's why WCAG limits flashing web content to at most three flashes per second.

What kind of content on websites typically flashes too fast?

Most often it's videos with strobe scenes, for example from concerts or parties, along with sweepstake animations, aggressive ad banners, and countdown effects with rapid colour changes. The actual page design is rarely the problem. It becomes critical with large-area, high-contrast changes in rapid succession — especially when intense red is involved.

Can an automated check detect flashing content?

A serious flash analysis would have to evaluate videos frame by frame for jumps in brightness, area coverage, and the proportion of red. Our audit service doesn't perform such a video analysis and therefore doesn't claim any judgment either: the report openly marks the criterion as a manual check point. For a health risk like this, an unsupported "passed" would be the worst possible answer.