Responsive design aims to keep a site usable across a range of viewport widths and orientations. Testing only one desktop and one phone width can miss overflow, wrapping, and navigation problems between those points. This browser-based tester resizes a sandboxed iframe to preset or custom CSS dimensions, making repeatable visual checks easy. It is a viewport-sizing aid, not a replacement for real-device testing, browser developer tools, or accessibility checks.
Choosing and Testing Breakpoints
Breakpoints are decisions made by a site's content and layout, not universal device categories. Tailwind, Bootstrap, and Material systems use different thresholds, and a site may need additional or fewer ones. Test the widths defined by the project's CSS, plus intermediate values just below and above each threshold. At each width, inspect navigation, columns, text wrapping, images, forms, and horizontal overflow. This tester can make those viewport checks repeatable, but it does not identify breakpoints or judge whether a layout is correct.
Presets vs Custom Viewport Testing
Presets are convenient named width-and-height pairs for common review targets. Custom dimensions are better when a project's CSS uses specific breakpoints or when you need to reproduce a bug report. The frame and iframe are resized, but the tester does not change the browser user agent, emulate a real device's DPR, draw its notch, provide safe-area insets, simulate its keyboard, or reproduce mobile browser chrome. Use browser device emulation and physical devices when those behaviors matter, then use this tester for quick viewport comparisons.
A Practical Responsive QA Checklist
Use this tester as a manual checklist: load a permitted URL, check narrow, intermediate, and wide widths, rotate mobile and tablet targets, and inspect text wrapping, images, forms, navigation, sticky elements, and horizontal overflow. A blank or error state may mean that the target blocks iframe embedding; it is not proof that the target itself is broken. Touch-target size, keyboard access, focus visibility, screen-reader behavior, and real-device issues need separate browser or assistive-technology checks. Record the width, height, zoom, browser, and target URL when filing a responsive bug so another person can reproduce it.