Module 3.2: Seizure Safety: Understanding the Flashing Content Rule

Why this rule exists

Most accessibility requirements ask you to add something — captions, alt text, transcripts. WCAG Success Criterion 2.3.1 is different. It asks you to remove something, or better yet, never include it in the first place.

The rule exists because certain visual patterns — specifically content that flashes rapidly — can trigger seizures in people with photosensitive epilepsy. A flashing logo intro or a strobing transition isn't just distracting for someone with this condition. It can cause a medical emergency. Photosensitive epilepsy affects roughly 1 in 4,000 people, and seizure triggers from flashing content have also been linked to headaches, dizziness, and disorientation in a broader population — including people who have never had a seizure before.

This is why WCAG classifies 2.3.1 as a Level A requirement — the baseline. It applies to every piece of web content you publish, and it's one of WCAG's non-interference criteria, which means it must be met regardless of whether other accessibility features are in place. The good news: most government video content doesn't come close to triggering it. But a few common production elements are worth a close look.

What the standard says

WCAG 2.3.1 states:

"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."

That "or" is important. The standard gives you two separate paths to pass. Content either stays under three flashes per second, or it stays below specific luminance and size thresholds. Understanding both helps you evaluate content accurately.

The three-flash rule

A flash is defined as a pair of opposing changes in relative luminance — a bright-to-dark shift followed by a dark-to-bright shift counts as one flash. Three of those in one second puts you at the threshold. Four or more means the content fails.

For most meeting recordings and presentation videos, this simply won't come up. A speaker at a podium doesn't flash. Static slides don't flash. Where you need to pay attention is in produced video elements: station IDs, animated logos, broadcast intros, motion graphics, and lower-third animations. Some of these were designed for television broadcast, where different standards apply.

The luminance threshold

Content can also pass based on size. WCAG specifies that flashing content is below the threshold if the combined area of flashes occurring at the same time covers less than 25% of any 10-degree visual field on screen. In practical terms, for content viewed at 1024 x 768 pixels at a typical viewing distance, that 10-degree field corresponds to roughly a 341 x 256 pixel rectangle anywhere on screen.

This is why not every sparkle or twinkle in a graphic is automatically a problem. Size and screen position matter alongside frequency.

The red flash rule

WCAG treats red flashing as a separate, heightened concern. A red flash is defined as any pair of opposing transitions involving saturated red, where one state has a red value in the CIE color model that meets the standard's saturation threshold.

Saturated red is especially effective at triggering photosensitive seizures, so highly saturated red transitions are flagged even when the overall luminance change might otherwise be within acceptable limits. In practice, be careful with flashing red alert graphics, rapid cuts involving bright red backgrounds, or pulsing red indicators in broadcast overlays.

What counts as a general flash

A general flash is a pair of opposing changes in relative luminance of 10% or more of the maximum relative luminance (1.0), where the relative luminance of the darker image is below 0.80. In plain terms: the contrast between the bright and dark states must be significant (at least 10%), and at least one of those states must be genuinely dark (below 0.80). Two very bright states flickering between each other are less hazardous than a stark bright-dark contrast. You don't need to calculate these numbers by hand — but understanding the concept helps you recognize which content deserves closer review.

Common government video content: where to look

Government media teams produce a consistent set of video elements across meetings, public access channels, and online archives. These are the areas where photosensitive content risks most often appear:

• Broadcast station IDs and channel bumpers. Many government public access operations run station ID videos produced years ago, sometimes by outside vendors or volunteers. These may include strobe effects, rapid cuts, and flashing text that would not support WCAG 2.3.1. If your channel runs IDs between programs, review them.

• Animated meeting intros. City council meetings, planning commission hearings, and other recorded sessions often begin with an animated title sequence. These were frequently produced as one-time design projects and may never have been evaluated for seizure safety.

• Lower-third graphics. Speaker identification overlays can include animated backgrounds or reveals. A lower-third that slides in with a bright flash, or features a rapidly pulsing design element, may need adjustment.

• Slide transitions. Presentation content recorded for public posting sometimes includes transition effects built into a template. Most modern defaults are fine, but "Flash" transitions (aptly named) or rapid dissolves between high-contrast slides warrant a second look.

• Stock footage and B-roll. Stock video sometimes includes content shot in environments with strobe lighting — industrial settings, emergency response scenarios — or uses editing techniques with rapid cuts.

How to test for compliance

Manual visual review

For most government video content, start by watching it at normal speed. Note any moments where you see rapid light changes, flashing, or high-contrast alternating patterns. If nothing stands out, the content is very likely fine. The three-flash rule is easy to check manually: if you can't count more than three flashes in any given second, you're at or under the threshold.

Automated analysis tools

For content where visual review leaves uncertainty, free analysis tools are available. PEAT (Photosensitive Epilepsy Analysis Tool) is a free application from the Trace Center that analyzes video files for flash frequency, luminance changes, and red flash thresholds. It generates reports that flag specific time codes where content may be problematic.

The Harding Test, developed for broadcast media compliance, uses similar analysis principles and is built into some professional video editing systems.

What to do when content fails

A note on warnings first: Some teams wonder whether adding a flashing content warning before a video counts as a fix. It doesn't. WCAG does not list a warning as a passing strategy for 2.3.1 — if content fails the threshold, it fails regardless of whether a warning is present. That said, a warning is a reasonable courtesy while you work toward a permanent fix.

When content does need remediation, your options are:

• Edit the source content. Work with whoever produced the content to modify the animation, remove the flash effect, or reduce the flash frequency. This fixes the problem at the source.

• Remove or replace the segment. For archived content that can't be re-edited, trim the problematic element from the web-published version. A station ID that fails the flash test can be cut before the main content begins.

• Slow down the animation. If an animated element flashes at four or five times per second, reducing the animation speed to under three flashes per second brings it into alignment with the criterion.

Real-world application: a county communications office

A county communications team records and archives all board of supervisors meetings for online public access. Their meetings begin with an animated intro the cable access studio produced in 2018. The intro includes a rapid zoom-in effect on the county seal that creates a series of light and dark flashes as the image expands and rotates.

When the communications coordinator runs the intro through PEAT, the tool flags a two-second segment where the zoom effect generates five general flashes per second. The rest of the intro is fine.

The coordinator works with the studio to revise the sequence — the zoom is slowed down and the high-contrast flash frames are removed. The new intro plays a smooth reveal of the county seal over a dark background. It's arguably more polished than the original, and it now passes analysis.

This kind of review-and-fix cycle takes an afternoon, not weeks. Most issues with flashing content in government media are isolated to specific produced elements, not spread throughout an entire video archive.

MediaScribe integration

MediaScribe's caption overlays display as static text that updates incrementally as words are transcribed. They render on fixed black-and-white backgrounds delivered through the Gateway appliance and browser-based interface, and do not include animated or flashing elements. This helps reduce the risk of the captioning layer introducing flashing content concerns.

When reviewing recorded meetings to support WCAG 2.3.1, your focus should be on the underlying video content — meeting intros, graphics packages, and video segments — rather than the caption display. MediaScribe's transcripts and caption files are available as static text documents, which are not subject to flashing content requirements.

Apply your 2.3.1 review to video files before they are published, as part of your standard content review workflow.

Summary

• WCAG 2.3.1 is a Level A, non-interference criterion — it applies to all web content regardless of other accessibility features.

• Content must not flash more than three times per second, or it must fall below the general flash and red flash luminance thresholds.

• A flash is a pair of opposing luminance changes — bright to dark, then dark to bright.

• Red flashing carries additional risk and uses a broader definition of what qualifies as a flash.

• The area of flashing content on screen matters — small elements are less likely to create concerns than large ones.

• Common risk areas in government media include animated intros, station IDs, lower-third graphics, and certain slide transitions.

• Manual visual review is the right starting point; automated tools like PEAT help when content is uncertain, and human review of flagged results is always recommended.