ONS Design System — complete local specimen

Implementation: Native ONS 73.13.1 Checkbox and Mutually Exclusive CSS/JavaScript. Original question, app validation/review and explicit Arial fallback.

1. Extract this archive into its own directory.
2. Run: python3 -m http.server 8080
3. Open: http://localhost:8080/ons.html

The HTML includes the document root, layout/theme overrides and initialization.
Its relative vendor directory includes the actual CSS, JavaScript module chain,
fonts and other packaged assets used by this specimen. Preserve this structure;
ES modules require HTTP rather than opening a file:// URL.

Dependencies included: ons-73.13.1
Official reference: https://service-manual.ons.gov.uk/design-system/components/mutually-exclusive

This is an original local demonstration, not an affiliated government service.
Actions have no external side effects. Retain supplied dependency licenses.
