Free Online Responsive Design Tester

1. Enter the URL you want to test.
2. Select a breakpoint preset: mobile, tablet or desktop.
3. Inspect how the page reflows at that width.
4. Switch presets to compare; note some sites block framing.

Responsive web design adapts a layout to the viewport using fluid grids, flexible images and CSS media queries — the approach formalised by the CSS Media Queries specification and the meta viewport convention. Designers reason about it in terms of breakpoints: widths at which the layout reflows, typically around 375px for phones, 768px for tablets and 1280px and up for desktops.

This tester loads a URL into an iframe sized to each preset width, so you can compare how a page reflows without resizing your whole browser. Switching presets is the quickest way to confirm that navigation collapses, columns stack and text stays legible at each tier.

One caveat: a site can forbid being framed by sending X-Frame-Options: DENY or a restrictive frame-ancestors directive, in which case its preview stays blank — that is a security policy, not a tool failure. The tester resizes the viewport visually only; it does not simulate touch, so verify gestures and device quirks on real hardware. Previews render in your browser.