Prototype HTML, CSS and JavaScript together: the three panes are assembled in the Worker into a complete document (title, viewport meta, optional CSS reset) and shown in the preview iframe, which for this tool runs scripts inside a sandbox with an opaque origin — no cookies, storage or access to the One Code page. A console overlay is injected so log/info/warn/error, uncaught errors and unhandled rejections appear at the bottom of the preview. External scripts and stylesheets are limited by the page CSP to the allowed CDN hosts, and the notes list what will load. Download the result as index.html.