Viewport Resizer

The Original Device Emulator

Viewport Resizer was among the first to bring responsive design emulation to the web. Released alongside the introduction of CSS media queries, it set an early standard for how digital interfaces could be tested across screen sizes. Originally launched as a universal browser bookmarklet, it reached over 1.2 million users within the first three months in 2012.

Such polish. Love it. — Paul Irish, Google Chrome

Its approach later became a foundational influence for Google Chrome’s built-in device emulation, which continues to be used by millions of users worldwide.

We came across your Viewport Resizer tool and really love it! — Mike Medric, Forbes Media

Today, the emulator returns in a redesigned, modern format — built for professionals working across an increasingly complex digital landscape. It offers a flexible, zoomable workspace to test and present websites across a wide range of devices, from smartwatches to ultra-wide monitors.

A useful responsive design testing tool. — Adobe

The environment is fully customizable, supporting real-time screen resizing, orientation changes, user-agent emulation, and precision tools like breakpoint guides and grids. Coffee Mode lets you sit back and watch as your design cycles through configured device presets automatically — ideal for relaxed overviews, showcases, or ambient testing.

This one by Malte Wassermann is a really good one. It goes the extra mile. — Chris Coyier, CodePen

Used by professionals, this emulator combines historical credibility with the needs of today’s responsive workflows.