See Your Website on Every Screen.
Explore responsive layouts, compare device sizes, and test supported interactions in one visual workspace.
Start by pasting any website URL above — no sign-up required.
Accelerate Your Team's Velocity
Seamless multi-viewport scaling tested simultaneously across modern browser breakpoints.
Watch Responsive Testing in Action.
See synchronized navigation, freeform canvas panning, and instant viewport rotation streamline responsive web testing in real time.
Synchronized Interactions
Scroll or navigate on one device and see all other active viewports mirror the action across compatible pages.
Infinite Spatial Workspace
Drag, arrange, and zoom in/out of viewports without disturbing layout state or reloading iframes.
Instant Viewport Rotation
Test landscape orientations on smartphones and tablets instantly — swaps width and height without reloading.
Everything You Need for Responsive Web Testing
From multiple device previews to synchronized interactions, Responsivelyonline makes it simple to test, compare, and perfect responsive designs.
All Devices at Once
Compare mobile, tablet, laptop, and desktop layouts side-by-side. Four default devices open immediately: iPhone 14 Pro (393×852), iPad Mini (768×1024), MacBook Pro (1440×900), and Desktop (1920×1080).
Spot layout differences instantly
Infinite Canvas
Pan, zoom, and position device frames freely on an open spatial workspace. Drag previews anywhere, zoom in to inspect details, or zoom out for a bird's-eye view of all viewports at once.
Open spatial layout — no fixed grid
Synchronized Interactions
Mirror supported scrolling, clicks, and navigation across compatible viewport frames. When you navigate on one device, other previews follow — where cross-origin security policies permit.
Test user flows across breakpoints
Instant URL Preview
Paste any URL and watch it load across all configured viewports simultaneously. The app auto-prefixes https:// (or http:// for localhost), validates the address, and checks embedding headers before loading.
See your site in every viewport immediately
Custom Viewports
Specify exact pixel widths and heights (e.g. 360×740, 1024×768) for edge-case breakpoints. Rotate supported devices between portrait and landscape without reloading the embedded page.
Test any breakpoint precisely
Device Library
Select from built-in presets including iPhone 14 Pro, iPad Mini, iPad Pro, MacBook Pro, and desktop monitors. Add fully custom dimensions for foldable screens or specific target devices.
Popular presets plus custom sizes
Preview Suites
Save device selections, zoom levels, orientations, and workspace theme to localStorage. Your preferred configuration persists between sessions so you can return to exactly your testing setup.
Persistent workspace preferences
Canvas Controls
Refresh all previews simultaneously, zoom all devices to a proportional fit, reset the layout, toggle compact mode, or switch between dark and light workspace themes — all from the top toolbar.
One-click workspace management
Route Navigation
Click links within a preview to navigate to new pages while the other viewports follow along. iframes are sandboxed to allow scripts, forms, and popups while preventing top-level navigation away from the workspace.
Inspect multi-page flows across sizes
Developer Workflow
Inspect your localhost or staging URL simultaneously across screen sizes in one workspace. In development mode, private and loopback addresses are fully accessible for local project testing.
Localhost support in dev mode
Your Responsive Testing Toolkit.
Every utility you need to inspect, rotate, measure, and validate responsive layouts is built right into the infinite canvas.
Device Presets
Instant selection of popular devices: iPhone 14/15, iPad Mini, iPad Pro, MacBook Pro, and standard desktop screens.
Custom Viewport Dimensions
Specify exact pixel widths and heights (e.g. 360 × 740, 1024 × 768) to test edge-case breakpoints and foldable screens.
Portrait & Landscape Rotation
Swap width and height dynamically on mobile and tablet frames without reloading the embedded document.
Zoom and Fit-to-Screen
Scale individual previews (50%, 75%, 100%) or compute a shared proportional fit scale across all active devices.
Refresh All Previews
One-click simultaneous reload for all active viewports to test code changes or verify fresh server state.
Synchronized Interactions
Mirror scrolling positions, click navigation, and form interactions between compatible viewport frames.
Infinite Canvas Navigation
Smooth spatial pan and zoom with interactive wave dot grid, device dragging, and automatic arrangement algorithms.
URL Loading & Normalization
Smart address formatting (auto-prefixing https:// or localhost http://) with X-Frame-Options embedding verification.
Add or Remove Devices
Flexibly expand your testing matrix from 1 up to 10+ devices, or trim down to inspect a single layout in detail.
Full-Screen Preview
Leverages the browser Fullscreen API on individual device previews for distraction-free UI evaluation.
How Responsivelyonline Works.
From pasting a link to comprehensive responsive audit in less than three seconds.
Enter a URL
Paste the website address into the preview input. Responsivelyonline automatically normalizes the URL scheme and verifies framing headers.
Choose Your Screens
View across multiple device dimensions simultaneously — phone, tablet, laptop, and desktop viewports side-by-side on an infinite canvas.
Explore and Compare
Rotate devices, inspect layouts, navigate the canvas with zoom and pan, and test synchronized scroll & click interactions.
Who Benefits from Responsive Testing.
Responsivelyonline fits naturally into daily development, QA, design, and agency workflows — anywhere viewport accuracy matters.
Frontend Development
Preview localhost or staging URLs across all breakpoints simultaneously while you code. Catch layout bugs before they reach production — no more toggling browser devtools.
Responsive Web Design
Verify that fluid grids, flexible images, and media queries behave as intended across mobile, tablet, and desktop widths. Inspect every breakpoint without resizing windows.
Landing Page Testing
Ensure your marketing pages look polished across all entry points. Compare hero sections, CTAs, and pricing layouts on every viewport size before launch.
Cross-Viewport QA
Run manual quality checks across multiple device sizes in one pass. Identify layout shifts, overflow issues, and clipped text faster than single-window inspection.
UI Regression Inspection
After a code change, refresh all viewports simultaneously and visually compare the same page across screen sizes side-by-side to spot unexpected regressions.
Website Redesigns
Compare the old and new design implementations at matching viewport sizes. Use the infinite canvas to position before/after versions next to each other.
Agency Client Reviews
Walk clients through responsive behavior by presenting all device views simultaneously. Rotate tablets, zoom into mobile, and demonstrate how layouts adapt on every screen.
Design System Validation
Check that design system components — typography scales, spacing, and component states — apply correctly across every configured viewport width.
One Workspace vs. Repeated Resizing.
Inspecting multiple viewports one at a time means you never see them all at once. A multi-viewport workspace changes how you review responsive layouts.
Without a Multi-Viewport Tool
Single browser window
- Open browser with dev tools
- Resize window to mobile width
- Check layout visually
- Resize to tablet, check again
- Resize to laptop, repeat
- Resize to desktop, repeat
- Navigate back, resize, re-check
- Lose track of which sizes you tested
Result: Linear, error-prone, slow — you never see all sizes together
With Responsivelyonline
Infinite canvas workspace
- Paste your URL into Responsivelyonline
- All four viewports load simultaneously
- Pan and zoom the infinite canvas freely
- Rotate devices to test landscape
- Refresh all previews in one click
- Synchronized scroll across viewports
- Add custom sizes for edge cases
- Every size inspected at the same moment
Result: All viewports visible simultaneously — easier comparison, faster review
Responsivelyonline is a manual inspection aid and does not replace automated testing, cross-browser test suites, or real-device hardware validation.
Resources for Better Web Experiences.
Essential architectural deep dives, viewport guides, and community engineering writeups.
Learnings That Led to Screenshot v2
An insightful look into how cross-origin iframe restrictions, browser security boundaries, and the Screen Capture API shaped modern viewport screenshot engines. Learn why region capture and pixel ratio adjustments are critical for crisp device exports.
Key Engineering Takeaways:
- Why browsers restrict direct pixel access across origin boundaries
- Implementing Chrome & Edge Region Capture for exact viewport bounds
- Handling pixel density multipliers on Retina and HiDPI screens
Understanding Iframe Sandboxing & CSP Framing Headers
How X-Frame-Options and Content Security Policy (frame-ancestors) protect web users, and how Responsivelyonline identifies framing policies upfront.
Contributing Custom Device Presets & Canvas Tools
Join web developers contributing to the Responsively open source ecosystem. Submit new device dimensions and viewport profiles.
Frequently Asked Questions
Everything you need to know about browser-based rendering, iframe sandboxing, device emulation limits, and more.
Still need help? Visit documentation or report an issue.
Bring Every Viewport Into Focus.
Start exploring responsive layouts from a single, flexible workspace.