Open index.html locally in a browser. Check or uncheck Hatch, Ring and Solid; the page announces the selected pass count, denominator and percentage. “Include all” restores 25/48≈52.1%; “Hatch + Ring only” shows 23/36≈63.9%. The original source table remains visible, so a learner can see that filtering changes scope, not evidence. If no family is chosen, the interface explains that no denominator exists. It uses no external assets, network request, account, cookie, analytics, personal data or save operation.
Use the paper denominator mat and its full text/tactile alternative for the same calculation. Keyboard checkboxes and buttons have native focus behaviour, visible outlines and an announced status. Browser accessibility still needs actual assistive-technology/user trial; this is a desk-checked implementation. The fictional register never establishes public accessibility or safety. All JavaScript is original Apache 2.0.