While the best way to test your responsive web designs is to use a variety of browsers and devices (including desktop, tablet, and smartphone), web developers often begin testing RWD by resizing the desktop browser. Browser resizing is handy but can sometimes be a little awkward. Check out the following online tools that will provide instant views of your web page in a variety of screen sizes and devices:
It's fun to view your responsive website on these browser tools. The true test, however, is to view your web pages on a variety of physical mobile devices.