Responsive Tool shows your site on mobile, tablet, and desktop at once — free, no signup, no download. Paste any URL, see it across real device viewports side by side. Swap any pane's device, enter custom widths, refresh one viewport after a code change, or share your setup via URL. Built with Next.js 16, React 19, Tailwind v4, on Cloudflare Workers. Includes reference pages for device viewport sizes and CSS breakpoints.