Responsive web design (RWD) is an approach to web design which makes web pages render well on a variety of devices and window or screen sizes. Recent work also considers the viewer proximity as part of the viewing context as an extension for RWD. Content, design, and performance are necessary across all devices. A site designed with RWD adapts the layout to the viewing environment by using fluid, proportion-based grids, flexible images, and CSS3 media queries.
The fluid grid concept calls for page element sizing to be in relative units like percentages, rather than an absolute unit like pixels or points. Flexible images are also sized in relative units, so as to prevent them from displaying outside their containing element.