It is a device-independent UI (user interface) design philosophy that aims to develop and deliver an optimized website experience on devices with different widths and different resolutions: PC, notebook, tablet, smart phone, etc. It is a technology using which web designers code the style sheets of the website in a manner that its layout magically adjusts itself to more comfortably fit the width of the browser in which it is being viewed.
Designed to work whether you are using a PC or a tablet, a smartphone, or a smart TV.
The latest standard for CSS which is completely backwards-compatible with earlier versions of CSS.
Produce faster-loading sites for access over cellular networks and deliver richer functionality avoiding some of the pitfalls of device-side-only solutions.
Flexible images are also sized in relative units to prevent them from displaying outside their containing element.