Responsive Tester - Mobile View, Breakpoint Checker & CSS Editor

Preview any page on multiple screens at once. Test mobile view, every screen size and breakpoint, and edit CSS live.

Responsive Tester is the fastest way to see how a page behaves at every screen size at once. Open any website in a row of synced viewports, phones, tablets, and desktops side by side, then click, scroll, and type in one screen and watch every other screen follow. Pick an element, edit its CSS live, and your changes land across every breakpoint instantly. Usage - Toggle the extension by clicking the icon, selecting from the right-click context menu, or using the shortcut Alt/Option + Shift + V. - Add viewports from built-in device presets, or enter custom dimensions. - Detect the site's own breakpoints and build matching viewports in one click. - Scroll, rotate, resize, or zoom any viewport, and capture a clean screenshot of any frame. - Turn on interaction sync to mirror clicks, hovers, typing, and navigation across every screen (Pro). - Open the editor, pick an element, and edit its CSS live across all breakpoints (Pro). Features - Preview any page across multiple synced viewports, in a scrolling row or wrapped into a grid. - Scroll sync keeps every viewport aligned, including scroll areas inside the page. - Built-in device presets for popular phones, tablets, and desktops. - Custom viewport dimensions. - Site breakpoint detection reads the page's own media rules and builds matching viewports. - Rotate any viewport between portrait and landscape. - Drag to resize viewports and test the widths in between. - Continuous zoom to fit more on screen. - One-click screenshots of any viewport. - Light and dark themes that follow your system or lock to either. - Works on any web page, including sites that normally refuse to load in frames. - Access from the toolbar icon, right-click context menu, or keyboard shortcut. - Customize keyboard shortcuts by visiting chrome://extensions/shortcuts. Responsive Tester PRO A PRO tier unlocks the two features that turn previewing into building. - Interaction Sync: Clicks, hovers, form input, key presses, and page navigation mirror across every viewport in real time, so you drive one screen and watch the whole matrix respond. Element matching is verified per interaction, so a synced action never lands on the wrong element. - Live CSS Editor: Pick any element and edit its rules in a side editor with full @media query support, so each change lands in the right breakpoint automatically. Force pseudo-states like :hover and :focus, undo and redo every change, and see edits apply instantly across every viewport at once. PRO requires a one-off payment for lifetime access, good for 3 activations. Every install includes a 7-day free trial with no card required, plus a 30-day money-back guarantee. Made For - Developers: Catch layout shifts and breakpoint bugs before they ship. Line up a phone, a tablet, and a desktop, then drive them all at once instead of resizing one window over and over. - Designers: Confirm a build holds up at every size and that the responsive behavior matches the intent, from the smallest phone to the widest desktop. Installation - Designed specifically for stability in Google Chrome 116+. - Installable on any Chromium browser; however, for the best experience, make sure your browser supports all the required APIs (like Offscreen). If there are any issues with installation, update your browser to the latest version and try again.