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.
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:
The fluid grid concept calls for page element sizing to be in relative units like percentages, rather than absolute units like pixels or points.
Flexible images are also sized in relative units, so as to prevent them from displaying outside their containing element.
Media queries allow the page to use different CSS style rules based on characteristics of the device the site is being displayed on, most commonly the width of the browser.
Responsive web design has become more important as the amount of mobile traffic now accounts for more than half of total internet traffic.
Mobile first, unobtrusive JavaScript, and progressive enhancement
“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.
Progressive enhancement based on browser, device, or feature detection
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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