Responsive Breakpoint Tester
Preview any URL or pasted HTML at mobile, tablet, laptop, and desktop widths with custom viewport sizes.
Preview layouts at real device widths
Responsive design depends on how content reflows at common breakpoints. Switch between mobile, tablet, laptop, and desktop widths, or set a custom viewport. Paste HTML for a sandboxed preview, or load an external URL when the site allows iframe embedding.
Related front-end tools
Iterate on markup in HTML Preview or validate structure with our HTML utilities. If a live site blocks embedding, use HTML paste mode with exported markup instead.
Frequently Asked Questions
Why is my URL preview blank?
Many sites send X-Frame-Options or Content-Security-Policy headers that block iframe embedding. Paste HTML instead.
Is JavaScript allowed in HTML mode?
Yes, within the iframe sandbox limits. Scripts run locally in your browser only.
Which breakpoints are included?
Presets cover 320px through 1280px, plus a custom width between 240px and 2400px.
Does this replace real device testing?
No. It is a quick width check. Always verify touch behavior and performance on physical devices when possible.
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