Responsive Viewer by Pixellize

About

Responsive Viewer by Pixellize is a powerful developer tool that lets you preview any website across multiple screen sizes — all at once, in a single tab. Whether you're building a landing page, testing a client's site, or debugging a layout issue, Responsive Viewer gives you an instant side-by-side view of how your design looks on mobile, tablet, laptop, and desktop — no resizing the browser window required. 
 
Simply click the extension icon on any webpage and an overlay launches instantly, loading the current URL across all your configured device sizes. The clean, distraction-free canvas keeps every preview in sync so you can spot breakpoint issues at a glance. Use the built-in URL bar to switch to any other site without leaving the viewer. 

Organize your workflow with a fully tab-based system. Create multiple named tabs — each with its own selection of screen sizes — so you can jump between a "Mobile-only" view, a "Full Device Set", or any custom combination in one click. Add, remove, or rearrange screens at any time using the Screens panel. 

Take full control with custom screen sizes. Beyond the built-in presets covering popular phones, tablets, and desktop resolutions, you can define any custom width and height (height defaults to 500 px if left blank). Your custom sizes are saved and available across all tabs.

The built-in Live CSS Editor lets you inject custom styles into every preview frame in real time — no page reload needed. The editor features syntax-aware autocomplete for CSS properties and values (including !important), auto-indentation, brace auto-closing, and line numbers. Close the sidebar to instantly remove the injected CSS and restore every frame to its natural state.

Need to inspect a single frame up close? Click the expand icon on any frame to zoom it to fill the entire canvas. The frame scales to fit perfectly while maintaining its exact device width. Click again to return to the full multi-device view.

Responsive Viewer works on virtually any website, bypassing iframe restrictions so even sites with strict security headers load cleanly inside the viewer. Your zoom level and custom devices are persisted across sessions so the tool is ready exactly where you left it.