Review deliberate lightning and strobe effects against the web content's flashing requirements before publication. A convincing scene or stable exposure is not evidence of conformity. Use the relevant WCAG 2.2 criterion and qualified analysis of the delivered version; if you cannot establish the result, reduce or remove the effect and reassess. This is a web accessibility workflow, not a guarantee of health safety.
Apply the actual web criterion
WCAG 2.2 Success Criterion 2.3.1 addresses content that flashes more than three times in any one-second period, unless it is below the general-flash and red-flash thresholds. A flash involves opposing transitions; counting bright frames or lamp appearances is not the same test.
The threshold definitions involve the brightness change, saturated-red behavior and combined flashing area. Three per second is not a license to guess using fixed one-second timeline blocks: a short burst can straddle their boundary. Review every relevant moving one-second interval, not just the average rate across a scene.
W3C's G15 technique describes using a tool when the simpler documented tests do not establish the result. Its linked analysis resources are examples, without W3C endorsement. No tool result or threshold calculation has been performed for the hypothetical cases in this guide.
Prepare the version and viewing conditions
Keep an original master and make a separate web delivery copy. List lightning, alarm lights, rapid bright-dark cuts and saturated-red effects as review candidates, even when they are intentional.
- Identify the exact final file, page placement and player options.
- Evaluate the largest viewing scale the site allows, including fullscreen where available.
- Include loop joins and repeated playback rather than analyzing one isolated cycle.
- Where applicable, use the provided version with the highest dynamic range.
- Have a competent accessibility reviewer use suitable analysis and document the scope and result.
A small embedded preview does not establish the result at fullscreen. Separate flashing objects also require consideration together when they flash concurrently. Do not invent an area percentage from the thumbnail or apply SDR assumptions to an unexamined HDR delivery.
A hypothetical lightning-loop revision
Imagine a website hero film with dramatic lightning near both the ending and beginning. A single cycle seems spaced out, but repeating it places the ending burst beside the next beginning burst. That join belongs in the assessment.
This is a hypothetical risk-review case, not a measured violation. One reversible option is a version with steady illumination or fewer, more separated changes while preserving the story through sound and reactions. Ask for reassessment of the actual revised loop; a slower-looking preview is not a recorded threshold result.
Record an appropriately narrow approval
Accept this checkpoint only when the applicable test supports the documented result for the delivered content and viewing conditions. Meeting this one criterion does not establish full WCAG conformity or safety for every viewer and display.
Warnings and a stop button do not substitute for resolving a threshold failure. If you cannot obtain credible analysis, remove the uncertain flashing effect before publishing that version. Keep the original, revised export and review record. Later changes to the cut, loop, color grade, dynamic-range version or presentation size require checking whether the earlier assessment still applies.
Sources
- W3C WCAG 2.2: Understanding SC 2.3.1 (updated September 17, 2025)
- W3C WCAG 2.2 Technique G15: Flash-Threshold Analysis
Prepared by Imagild Editorial with AI assistance and reviewed against the linked sources on October 13, 2026. Examples are hypothetical; external procedures do not establish Imagild features or guarantee results.