4 Chrome extensions for responsive testing

Fact-checked 14 September 2026

Compare layouts, resize the browser, or prepare a device preview without switching to a separate desktop application.

Responsive testing extensions take different approaches. Some put several viewports on one canvas. Others resize the browser window or wrap the page in a device mockup. Start with the task you repeat most, then choose the setup that makes that task easier.

Published by Responsive Tester. Product descriptions are checked against the linked official sources. Suggested uses are editorial judgments, not benchmark results.

Compare the approaches

OUR PICK

1. Responsive Tester: compare and refine layouts

Responsive Tester opens multiple viewports side by side. You can set custom dimensions, save viewport sets as workspaces, and scroll them together. Its Site Breakpoints tool reads supported min-width and max-width media queries from accessible stylesheets and can build viewports from the detected thresholds.

Pro adds interaction sync, inspection, and live CSS editing. Select an element and edit its rules while keeping the other widths visible. A rule inside a media query still follows its own condition, which helps you spot a fix that works at one breakpoint but creates a problem at another.

Viewport management, scroll sync, breakpoint detection, overflow warnings, and screenshots are free. The seven-day trial includes the Pro tools. See the setup documentation, breakpoint detection details, and plan comparison.

Choose it for: an editing loop where you want to compare the result of a CSS change across several widths immediately.

2. Responsive Viewer: a canvas for visual review

Responsive Viewer provides a multi-screen canvas with device presets, custom dimensions, several layout arrangements, and synced scrolling and navigation. Its official site also describes screenshot and annotation tools for sharing feedback.

The product has a free offering and a paid Professional subscription. The listed paid benefits include removing ads, access to all device mockups and screens, and screenshot annotations. Check the official feature and plan overview for the current split.

Choose it for: organizing several layouts for review, particularly when device mockups and annotated captures are part of the handoff.

3. Window Resizer: resize the browser you already use

Window Resizer by CoolX10 changes the browser window's dimensions using a customizable preset list. Its Chrome Web Store listing says presets can target the whole window or just the viewport. It also documents configurable global shortcuts and settings export and import.

Choose it for: checking a page at repeatable sizes in the browser window itself. This is a different workflow from comparing many embedded previews: pick a size, inspect the page, then move to the next size.

When testing a media-query threshold, target the viewport dimensions rather than assuming the outer window has the same width. Browser controls take space. The official store listing describes the sizing options, and the developer's site covers its live measurement display.

4. Mobile FIRST: review and present device previews

Mobile FIRST, the mobile simulator from webmobilefirst.com, offers device presets and custom devices. Its official site describes side-by-side device views with shared navigation, screenshots with transparent backgrounds, and video capture.

Choose it for: reviewing mobile layouts and preparing captures for a presentation or client discussion. A device-shaped preview can help communicate the intended context, while custom dimensions let you check widths beyond the preset list. See the official product overview for the features and installation link.

A device mockup or user-agent setting does not turn desktop Chrome into the browser engine running on that phone. Use the preview to review layout, then verify important flows on the actual browser and device.

Do you need an extension?

For occasional checks, Chrome's built-in Device Mode lets you enter viewport dimensions, choose presets, and display media-query breakpoints. It also includes CPU and network throttling. See the official Device Mode guide.

If you prefer a separate responsive development workspace, Responsively App and Sizzy are desktop options. They belong in a different category from Chrome extensions; the broader testing tools comparison covers them.

Try the same task in each extension

1. Open a page you actually maintain, including its normal sign-in state if needed.
2. Check a narrow width, a wide width, and the exact breakpoint where the layout changes.
3. Open the navigation and a form or dialog. Review those states as well as the initial page.
4. Resize between the presets and check for clipped content or accidental horizontal scrolling.
5. Capture an issue or change a CSS value, depending on the workflow you want to improve.

Pick the extension that makes your repeated work simpler. You still need checks in other browser engines and on physical devices: a wall of Chrome viewports is useful layout coverage, not complete cross-browser testing.

Continue with the responsive testing checklist, or compare live CSS editing extensions if editing is your main priority.