---
name: Flashing content
slug: flashing-content
category: accessibility
status: published
created: 2026-08-21T00:00:00.000Z
modified: 2026-08-21T00:00:00.000Z
definition: Content that flashes more than three times per second in a large
  enough area, restricted because it can trigger seizures.
aliases:
  - name: three flashes threshold
    source: wcag
  - name: photosensitive epilepsy
    source: community
  - name: general flash and red flash thresholds
    source: wcag
  - name: seizure safe
    source: community
tags:
  - perception
  - wcag
relations:
  contrastWith:
    - prefers-reduced-motion
    - pause-stop-hide
  variantOf: []
  partOf: []
  seeAlso: []
implementations: []
sources:
  - title: "WCAG 2.2: Three Flashes or Below Threshold"
    url: https://www.w3.org/TR/WCAG22/#three-flashes-or-below-threshold
demo: inline
exhibit: false
useWhen: judging whether a flash rate is dangerous
---

Success criterion 2.3.1, Three Flashes or Below Threshold, is Level A and it is the one criterion
in the standard written to prevent physical harm. Nothing on a page may flash more than three
times in any one second, unless the flashing stays below the general flash and red flash
thresholds. The risk is a seizure in a reader with photosensitive epilepsy, which is why this
criterion has no user preference attached to it, no opt-in, and no severity discussion: three
per second is the line, and 2.3.2, Three Flashes, at AAA, removes even the threshold exemption.

The thresholds are what make the rule usable rather than absolute. A flash is a pair of opposing
changes in relative luminance, and the general threshold is crossed when that change is 10
percent or more of maximum relative luminance while the darker state sits below 0.80. Saturated
red gets its own rule, because transitions to and from deep red are more provocative than the
same change in any other hue. And there is a small safe area: flashing is exempt if the flashing
region is no larger than roughly a quarter of a small central area of vision, which works out at
about 21,824 pixels, a region around 341 by 256, at a typical viewing distance. Three numbers,
then: how fast, how bright a swing, and how big.

The failures in ordinary product work are rarely deliberate. An autoplaying hero video with a
strobe cut, a glitch effect looping in a background, a loading state that inverts, a theme that
flips back and forth while a preference resolves, a scroll-driven animation that stutters into a
flicker when a heavy page drops frames. Note that
[prefers reduced motion](/prefers-reduced-motion) does not discharge this duty. A preference is a
standing request from the readers who made it, and this criterion protects everyone, including
the reader who has never touched that setting. A slow
[pulse animation](/pulse-animation) is not in scope at all, because it is one cycle every second
or two and a small swing at that, and where the movement is long-running rather than dangerous
the applicable rule is [pause, stop, hide](/pause-stop-hide).

The specimen above runs a flash faster than the count and stays lawful, which is the part worth
seeing. Its lamp swings between white and a very light grey four times a second: past three, and
still not a flash, because the swing is 13.7 percent of maximum while the darker state sits at
0.86, and a general flash needs both a swing over 10 percent and a darker state under 0.80. The
lamp is also 6,720 pixels, which puts it inside the small safe area on its own. Two exemptions,
either of them sufficient. The third control asks for twenty four a second at full contrast over
a large region, and the specimen refuses that one: it holds a still frame and shows the numbers
it would be failing. Explaining a seizure risk by producing one, unattended, on a loop, beside an
article somebody is reading, is the single thing this entry must not do, and the way out of that
bind is not silence but measurement.