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Responsive Breakpoint Previewer

Preview any URL at common responsive breakpoints (mobile, tablet, desktop) side by side or individually in an iframe.

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About this tool

Responsive Breakpoint Previewer loads a public URL in independently scrollable sandboxed iframes at four fixed viewport widths: 375, 768, 1024, and 1440 pixels. Toggle frames to compare layouts side by side. The preview cannot bypass a site's X-Frame-Options or Content-Security-Policy frame restrictions, and localhost or private URLs are not publicly reachable by the browser context.

Why use it

  • See multiple breakpoints simultaneously without resizing your browser or opening DevTools.

  • Includes presets for Tailwind, Bootstrap, and Material breakpoints — match your framework's tokens.

  • Each frame is individually scrollable for full page inspection.

  • Works with localhost and any publicly accessible URL.

How to use

  1. Enter the URL of the page you want to preview in the address bar.
  2. Click Preview to load it in the breakpoint frames.
  3. Use preset breakpoints or enter custom pixel widths.
  4. Click any frame to expand it to full view for detailed inspection.
  5. Use preset breakpoints (Tailwind, Bootstrap, Material) or enter custom pixel widths.
  6. Toggle between 'all visible at once' grid and 'one at a time' carousel modes.
  7. Reload all frames simultaneously when you push code changes to localhost.

When it helps

  • Quickly checking a responsive design at multiple breakpoints during development.
  • Comparing mobile and desktop layouts side by side for a client review or stakeholder demo.
  • Verifying breakpoint changes after editing CSS media queries.
  • Testing a staging URL at device-specific widths before launch.
  • Reviewing a competitor's responsive design quickly across viewports.

Examples

Tailwind preset comparison

Input
URL: https://example.com — preset: Tailwind
Expected result
Frames at 640px (sm), 768px (md), 1024px (lg), 1280px (xl), 1536px (2xl) loaded simultaneously.

Localhost dev server

Input
URL: http://localhost:3000 — widths: 375, 768, 1280
Expected result
Three frames preview your local Next.js or Vite app at iPhone, iPad, and desktop widths.

Custom edge cases

Input
URL: staging.example.com — widths: 320, 412, 1920
Expected result
Tests narrow Android (320), modern Pixel (412), and full HD (1920) — covers extremes traffic actually sees.

Blocked iframe site

Input
URL: https://accounts.google.com
Expected result
Frames render blank — Google sets frame-ancestors 'self', so external preview is impossible. Use DevTools instead.

Tips

  • When testing localhost, use 0.0.0.0 instead of localhost if you need to load the URL from another device — but the iframes load the same browser context, so localhost works fine here.
  • Add 320px (smallest common mobile) and 1920px (full HD) as custom widths to bracket your real-world traffic.
  • Some sites set frame-ancestors CSP headers and refuse to load in iframes — major sites like Google, Facebook, and bank logins are blocked. Test those in DevTools instead.
  • Use the 'reload all' button after deploying so every frame refreshes against the new code in one click.
  • If a frame shows scrollbars at all widths, your page has horizontal overflow — investigate min-width or fixed-width content.
  • Test landscape phones (812px width) too — many designs only check portrait widths and break in landscape.

Frequently Asked Questions

Can I preview localhost URLs?
Yes. As long as your browser can access the URL, the previewer can load it. localhost and 127.0.0.1 addresses work when running from the same machine.
Why might some pages not load in the frames?
Pages with X-Frame-Options: DENY or Content-Security-Policy: frame-ancestors 'none' headers refuse to be embedded in iframes. Many major websites use these headers.
Does it simulate touch events?
No. The frames are iframes at the specified width — touch event simulation and media queries responding to pointer type require DevTools device emulation.
What breakpoints does Tailwind use?
Tailwind v3 default breakpoints: sm=640px, md=768px, lg=1024px, xl=1280px, 2xl=1536px.
Can I add a custom breakpoint?
Yes. You can enter any custom pixel width in addition to the preset breakpoints.
Does this account for device pixel ratio?
No. Iframes render at CSS pixel widths regardless of your monitor's DPR. For DPR-sensitive testing (retina image swaps), use DevTools device emulation.

Explore the category

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Glossary

Breakpoint
A viewport width threshold at which a CSS layout changes — typically expressed as min-width or max-width media queries.
Media query
CSS @media rule that applies styles only when conditions are met (viewport width, orientation, prefers-color-scheme, etc.).
Mobile-first design
Authoring CSS for the smallest viewport first, then layering larger-breakpoint overrides via min-width media queries.
Viewport
The visible area of the browser window (or iframe). Measured in CSS pixels, not device pixels — separate from physical screen resolution.
Device pixel ratio (DPR)
Ratio of physical pixels to CSS pixels. Retina screens have DPR 2 or 3 — important for image sourcing but transparent for layout.
Tailwind breakpoints
Default Tailwind v3 breakpoints: sm=640px, md=768px, lg=1024px, xl=1280px, 2xl=1536px. Customizable in tailwind.config.js.
X-Frame-Options / frame-ancestors
HTTP headers that prevent a page from being embedded in iframes. Many major sites set these for clickjacking protection — those URLs cannot be previewed.
Container query
Modern CSS feature that styles based on the size of a parent container rather than the viewport. Complements but doesn't replace media queries.