Generates a tester page that frames the URL at real device viewport sizes — phone, tablet and desktop presets or a custom size — scaled to fit, with a toolbar to switch URLs, reload, rotate and zoom. It runs in the preview and downloads as one HTML file. Sites decide whether they may be embedded: pages sending X-Frame-Options or CSP frame-ancestors stay blank, and inside the sandboxed preview framed sites have no cookies or storage, so use the downloaded page for logged-in views. The notes list which Tailwind and Bootstrap breakpoints the chosen widths cross and a mobile checklist.