What is FlexViewport?
FlexViewport is a free online tool for testing responsive websites across several CSS viewport sizes at once. Compare phone, tablet, laptop and desktop layouts side by side.
Do I need to register?
The core responsive tester is currently available for free without registration. Enter a website address to open the workspace; no account is required.
Which sizes can I test?
Choose catalog presets or enter a custom width and height. The catalog includes iPhone, Android, tablet and desktop sizes. You can open up to eight viewports at once. Dimensions are measured in CSS pixels.
Can I rotate a viewport or change zoom?
Yes. Rotation swaps width and height, while zoom only scales the displayed preview. At 50% zoom, for example, the iframe still lays out content at its original CSS width. The catalog DPR is descriptive metadata, not an emulation setting.
Why do some websites not appear?
The target website opens in an iframe in your browser. Its X-Frame-Options or CSP frame-ancestors policy may forbid embedding. This website may block embedding in an iframe. FlexViewport does not bypass these restrictions or identify blocking from a load event; open the address in a new tab. An HTTP page may also be blocked within an HTTPS site.
Where are URLs and preferences stored?
The tester code does not send the target URL to the FlexViewport server. The full address stays in memory for the current test. Local browser history stores up to 20 origins without paths, queries or fragments and can be cleared. Viewport preferences and theme are also stored locally. Target sites receive iframe requests directly from your browser. GTM is connected for analytics; events defined in the code exclude target URLs, history and custom device names. See the privacy notice for details.
What does this test not replace?
A preset sets the iframe size; it does not run the device OS, Safari or touch emulation. There are no server screenshots, automatic quality scores or synchronized scrolling of external sites. Check browser-specific behavior and input on real devices before release.