Responsive web design (RWD) or responsive design is an approach to web design that aims to make web pages render well on a variety of devices and window or screen sizes from minimum to maximum display size to ensure usability and satisfaction.

A responsive design adapts the web-page layout to the viewing environment by using techniques such as fluid proportion-based grids, flexible images, and CSS3 media queries, an extension of the @media rule, in the following ways:

Responsive web design became more important as users of mobile devices came to account for the majority of website visitors. In 2015, for instance, Google announced Mobilegeddon and started to boost the page ranking of mobile-friendly sites when searching from a mobile device.

Responsive web design is an example of user interface plasticity.

Luke Wroblewski has summarized some of the responsive and mobile design challenges and created a catalog of multi-device layout patterns. He suggested that, compared with a simple responsive web design approach, device experience or RESS (responsive web design with server-side components) approaches can provide a user experience that is better optimized for mobile devices. Server-side CSS generator implementation of stylesheet languages like Sass can be part of such an approach. Google has recommended responsive design for smartphone websites over other approaches.

Although many publishers have implemented responsive designs, one challenge for responsive design adoption was that some banner advertisements and videos were not fluid. However, search advertising and (banner) display advertising came to support specific device platform targeting and different advertisement size formats for desktop, smartphone, and basic mobile devices. Different landing page URLs have been used for different platforms, or Ajax has been used to display different advertisement variants on a page. CSS tables permitted hybrid fixed and fluid layouts.

There have been many ways of validating and testing responsive web designs, ranging from mobile site validators and mobile emulators to simultaneous testing tools like Adobe Edge Inspect. The Chrome, Firefox and Safari browsers and developer tools have offered responsive design viewport resizing tools, as do third parties.

The W3C specification of HTML+ stated that websites have to be rendered according to the user preferences. The customization of web page layout was lacking however. Many web developers resorted to ordinary HTML tables as a way to customize the layout and bring some basic responsiveness to their websites at the same time.