Responsive design

Responsive web design (RWD) is an approach to web design aimed at allowing desktop webpages to be viewed in response to the size of the screen or web browser one is viewing with. In addition it's important to understand that Responsive Web Design tasks include offering the same support to a variety of devices for a single website. As mentioned by the Nielsen Norman Group: content, design and performance are necessary across all devices to ensure usability and satisfaction.

A site designed with RWD adapts the layout to the viewing environment by using fluid, proportion-based grids, flexible images, and CSS3 media queries, an extension of the @media rule, in the following ways:

Responsive web design has become more important as the amount of mobile traffic now accounts for more than half of total internet traffic. Therefore, Google announced Mobilegeddon (April 21, 2015) and started to boost the ratings of sites that are mobile friendly if the search was made from a mobile device. Responsive web design is an example of user interface plasticity.

"Mobile first", unobtrusive JavaScript, and progressive enhancement are related concepts that predate RWD. Browsers of basic mobile phones do not understand JavaScript or media queries, so a recommended practice is to create a basic web site and enhance it for smart phones and PCs, rather than rely on graceful degradation to make a complex, image-heavy site work on mobile phones.

Where a web site must support basic mobile devices that lack JavaScript, browser ("user agent") detection (also called "browser sniffing") and mobile device detection are two ways of deducing if certain HTML and CSS features are supported (as a basis for progressive enhancement)—however, these methods are not completely reliable unless used in conjunction with a device capabilities database.

For more capable mobile phones and PCs, JavaScript frameworks like Modernizr, jQuery, and jQuery Mobile that can directly test browser support for HTML/CSS features (or identify the device or user agent) are popular. Polyfills can be used to add support for features—e.g. to support media queries (required for RWD), and enhance HTML5 support, on Internet Explorer. Feature detection also might not be completely reliable; some may report that a feature is available, when it is either missing or so poorly implemented that it is effectively nonfunctional.

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Wikipedia