Photosensitivity & Safety

The Web Content Accessibility Guidelines (WCAG), published by the W3C, define a flash as "a pair of opposing changes in relative luminance that can cause seizures in some people if it is large enough and in the right frequency range." The W3C's own summary is blunt: "Flashing content can cause migraines, dizziness, nausea, and seizures."

Photosensitive epilepsy is the condition in which seizures are triggered this way. The Epilepsy Society describes it as seizures triggered by flashing lights or contrasting light and dark patterns, and puts it at around 3% of people with epilepsy; Epilepsy Action gives 3 to 5%. The Epilepsy Society says flashes between 3 and 30 a second are the common triggers, and Epilepsy Action that 10 to 25 a second is the most likely range; both say some people are sensitive at rates outside those ranges, up to 60 a second.

Flashing can also make people feel unwell who do not have epilepsy at all. The Epilepsy Society notes that feeling disoriented or uncomfortable around flashing does not by itself mean someone has photosensitive epilepsy.

WCAG 2.1 success criterion 2.3.1, "Three Flashes or Below Threshold", at level A, reads: "Web pages do not contain anything that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds."

The threshold is about area. WCAG's definition says flashing passes when "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", and gives a practical measure: "a 341 x 256 pixel rectangle anywhere on the displayed screen area when the content is viewed at 1024 x 768 pixels."

The W3C adds a warning that shaped how the one flashing tool here works: turning a flash off "is also not an option since the seizure could occur faster than most users could turn it off." A stop button is not a safeguard on its own. Keeping the flashing small is.

The Stuck Pixel Fixer cycles colors in a small square, for people who want to try it on a stuck pixel. It keeps these limits:

• Warning first. Nothing opens until you tick "I understand" under the seizure warning. If your device asks for reduced motion, the warning says so again and a second tick is required.
• Small area only. The square is at most 100 × 100 pixels, never full screen, and far smaller than WCAG's 341 × 256 area.
• A capped rate. The color changes no more than eight times a second. That is inside the range the charities describe, which is why the rate is not the safeguard: the small area is.
• You start it, and it stops easily. You place the square while it shows one steady gray, then press Start. Any key, click or tap, switching tabs or leaving the window stops it at once, and it stops by itself after 10 minutes.

These limits are tested automatically: a check measures the brightness of every part of the screen, frame by frame, and fails if anything changes outside the square, if the square is larger than 100 × 100 pixels, or if it changes more than 10 times in any second.

No other tool on ScreenTail is designed to flash. Moving screens start only when you press a button, and if your device asks for reduced motion they show a still picture instead. Even the Screen Flicker Test shows only steady fields: your phone's camera does the looking.

Every moving tool is checked before it is published by the same kind of detector, which fails a tool if any part of the screen jumps sharply in brightness from one frame to the next. It is a fault detector, not a safety guarantee. How We Test describes it, and the Accessibility Statement covers the rest of what the site does.

Stop, and look away from the screen. Both charities give the same advice for a possible trigger that appears without warning: cover one eye completely with the palm of your hand and turn away, and do not close your eyes, which they say can itself cause a flicker effect. If you think you have had a seizure, or you feel unwell afterwards, get medical help.

This page is information, not medical advice. For advice about your own health, talk to a doctor. If something on ScreenTail flashed when it should not have, write to contact@screentail.com with the page's address, and it will be looked into.