See the same page side-by-side at phone, tablet, and desktop widths — paste your HTML (or point at a URL), toggle devices, rotate them, and scroll every frame in sync. Everything runs in your browser.
Draft restored from this browser — “Clear HTML” to start over.
Heads-up: many sites (Google, GitHub, most apps with logins) send X-Frame-Options / CSP headers that tell the browser to refuse embedding, and there is no client-side workaround. Cross-origin frames are also sealed off from this page, so I can't reliably tell "loaded fine" from "blocked" — if the frames stay blank or show an error, the site is refusing to be framed. For those pages, save the HTML and use the Paste HTML tab instead. Sync scrolling and the dark hint are also unavailable cross-origin.