Responsive web design (RWD) is a setup where the server always sends the same HTML code to all devices and CSS is used to alter the rendering of the page on the device.
Responsive design serves all devices with the same code that adjusts for screen size.
Pages optimized for a variety of devices must include a meta viewport element in the head of the document. A meta viewport tag gives the browser instructions on how to control the page’s dimensions and scaling.
The meta viewport tag gives the browser instructions on how to adjust the dimensions and scaling of the page to the width of the device. When the meta viewport element is absent, mobile browsers default to rendering the page at a desktop screen width (usually about 980px, though this varies across devices). Mobile browsers then try to make the content look better by increasing font sizes and either scaling the content to fit the screen or showing only the part of the content that fits within the screen.