Make your design usable on devices with screens of all sizes, from small phones to large monitors.
Responsive design allows websites to be usable and well organized on all device sizes, from small phones to large monitors.
Media Queries allow for different style rules to be applied based on the size of the screen. Using them well is fundamental to implementing responsive design.
Several tools are available for testing responsive designs. Many of them are available in common browsers such as Firefox and Chrome.