Area Flash Checker - WCAG Flash & Seizure Safety Testing (2.3.1 / 2.3.2)

Measure WCAG flashing (SC 2.3.1 / 2.3.2) right in the browser. Select an area, capture 5-60 s, and count general and red flashes.

Area Flash Checker measures flashing content (WCAG Success Criteria 2.3.1 / 2.3.2) right in the browser, on the live page. Flash testing has usually required a dedicated video analyzer such as PEAT: screen-record the page, save the file, load it into the tool, and repeat the whole cycle after every fix. Area Flash Checker removes that detour. Drag to select an area (or check the whole viewport), capture it for 5 to 60 seconds, and the tool analyzes the actual rendered pixels frame by frame to count general and red flashes exactly as the user sees them. Video, as well as CSS animations, Canvas, carousels and ad units: anything that moves on the page can be checked where it runs. 🔍 How to Use 1. Click the extension icon to start. 2. Drag to select the area you want to check, or press F to select the whole viewport. 3. Click "Start" to capture (10 s by default, adjustable from 5 to 60 s). Hold Alt (Option on Mac) beforehand to interact with the page and trigger the animation you want to measure. 4. The result card shows the WCAG verdict, flash counts, and a timeline of what happened. ⭐ Key Features ▪️ Live, In-Place Measurement No screen recording, no video upload. CSS animations, video, Canvas, GIFs, carousels, and ad units are analyzed as they actually move on the page, using the tab's rendered frames. ▪️ WCAG SC 2.3.1 / 2.3.2 Verdicts Pass/fail for Three Flashes or Below Threshold (Level A) and Three Flashes (Level AAA), evaluated separately for general flashes and red flashes. - General flash: a pair of opposing relative-luminance changes of 0.1 or more where the darker state is below 0.80. - Red flash: the definition used since WCAG 2.1 (transition to or from a saturated red where R/(R+G+B) ≥ 0.8, with a CIE 1976 UCS chromaticity difference greater than 0.2). - Area threshold: 21,824 CSS px² (25% of a 341×256 px region), the value WCAG recommends. Alongside the verdict, the result card reports the maximum flashes per second, the violating area versus the threshold, and the time ranges where the limit was exceeded, with the peak one-second window called out. ▪️ Timeline and Flash Map A timeline plots luminance change, flashing area, and violating area over the capture. Hover or use the arrow keys to scrub, and a frame preview shows which parts of the selected area were flashing at that moment. ▪️ Built for Real Pages - Requests 60 fps capture and shows the effective frame rate. A warning appears if the rate drops low enough to miss fast flashing. - Retina / High-DPI and browser zoom are handled automatically. - Thin elements such as hairlines and text edges can be excluded from the area calculation (default 40 px; adjustable). - Capture duration (5–60 s) and the thin-structure filter are configurable in Settings. - Escape backs out from any step. The measuring overlay never appears in the captured frames. - Works with screen readers: measurement state and results are announced. 📊 Comparison with Other Tools Flash-testing tools have almost all been video-file analyzers such as PEAT: you record the page, save a file, and hand it to a separate tool. Very few can measure moving content right in the browser. Area Flash Checker removes that detour and completes the check inside the browser. - No recording: measure what is moving on the page, while it moves. - Target precisely: check just the element in question instead of the whole screen. - Video included: embedded and background video are measured as they actually play on the page. For auditing a finished video file itself, a video-file analyzer is the better fit. ⚠️ Disclaimer Results are an approximation based on the tab capture that the browser provides. The effective frame rate depends on your machine and the page; flashing faster than roughly half the effective frame rate may be missed or undercounted. Results may differ from video-file analyzers, and the tool does not detect spatial patterns such as stripes or checkerboards. This extension does not guarantee final accessibility compliance decisions. The capture must be started from the extension icon, and it does not run on browser-internal pages (chrome://) or the Chrome Web Store. 💻 Supported Browsers Google Chrome 116 or later. The extension relies on the tabCapture and Offscreen APIs; other Chromium-based browsers are not verified. 🔒 Privacy We do not collect any data. The tool makes no network requests. Captured frames are analyzed in the browser's memory and discarded immediately; the small preview frames shown on the result card are kept only while the card is open and are never saved or transmitted.