Responsive Breakpoint Tester
Enter a URL and instantly preview it at standard responsive breakpoints — mobile (375px), tablet (768px), laptop (1024px), and desktop (1440px). Use the custom width input for precise testing.
Frequently Asked Questions
Why does some sites show blank in the preview?▼
Many websites use X-Frame-Options: DENY or Content-Security-Policy: frame-ancestors 'none' HTTP headers to prevent embedding in iframes. This is a security measure to prevent clickjacking attacks. If the page shows blank, the site is actively blocking iframe embedding. To test those sites, use Chrome DevTools device toolbar (F12 → toggle device) or the Responsive Design Mode in Firefox DevTools.
What are the standard responsive breakpoints?▼
Common breakpoints: 320px (small mobile), 375px (iPhone standard), 768px (tablet/iPad portrait), 1024px (tablet landscape / small laptop), 1280px (laptop), 1440px (desktop), 1920px (wide desktop). Tailwind CSS uses: sm=640px, md=768px, lg=1024px, xl=1280px, 2xl=1536px. Bootstrap 5 uses: sm=576px, md=768px, lg=992px, xl=1200px, xxl=1400px.
What viewport width should I optimize for first?▼
Design mobile-first. As of 2024, ~60% of global web traffic comes from mobile devices. Start with 375px (iPhone standard), ensure usability at 390px (iPhone 14/15 Pro), then progressively enhance for tablet (768px) and desktop (1024px+). Focus most optimization effort on the 375-430px range for the best mobile coverage.
How do I test responsive design without a tool?▼
In Chrome DevTools (F12), click the device toolbar icon or press Ctrl+Shift+M (Cmd+Shift+M on Mac). This gives you a responsive emulator with preset devices and custom dimensions. Firefox has a similar Responsive Design Mode (F12 → device icon). Both tools also simulate touch events, device pixel ratios, and network throttling.
What is the difference between viewport width and screen resolution?▼
Screen resolution is the physical pixel count of the display (e.g. 2560×1440 for a 27" 1440p monitor). Viewport width is the CSS pixel width the browser uses for layout (e.g. 1440px). On high-DPI (Retina) displays, screen resolution is 2-3× the CSS viewport. Use CSS pixels for responsive breakpoints, not physical pixels.
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