The main development methodology behind responsive design is the use of media query functionality in CSS3. Media queries target not only certain device types (e.g., Android vs. iPhone), but actually inspect the physical characteristics of the device that renders the page. For example, the code below “asks” the device if its max horizontal resolution is equal to 480px: <link rel="stylesheet" type="text/css", media="screen and (max-device-width: 480px)", href="http://uxmag.com/iphone.css" /> If it is 480px, the device will load iphone.css. If not, the link is ignored. You can also include media queries in CSS as part of an @media rule: