Responsive web design
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Responsive web design (RWD) is an approach to web design aimed at crafting sites to provide an optimal viewing and interaction experience—easy reading and navigation with a minimum of resizing, panning, and scrolling—across a wide range of devices (from desktop computer monitors to mobile phones).
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 is becoming more important as the amount of mobile traffic now accounts for more than half of total internet traffic.  This trend is so prevalent that Google has begun to boost the ratings of sites that are mobile friendly if the search was made from a mobile device.  This has the net effect of penalizing sites that are not mobile friendly.
- Progressive enhancement based on browser, device, or feature detection
Challenges, and other approaches
Luke Wroblewski has summarized some of the RWD and mobile design challenges, and created a catalog of multi-device layout patterns. He suggests that, compared with a simple RWD 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 "dynamic CSS" implementation of stylesheet languages like Sass or Incentivated's MML can be part of such an approach by accessing a server based API which handles the device (typically mobile handset) differences in conjunction with a device capabilities database in order to improve usability. RESS is more expensive to develop, requiring more than just client-side logic, and so tends to be reserved for organizations with larger budgets. Google recommends responsive design for smartphone websites over other approaches.
Although many publishers are starting to implement responsive designs, one ongoing challenge for RWD is that some banner advertisements and videos are not fluid. However, search advertising and (banner) display advertising support specific device platform targeting and different advertisement size formats for desktop, smartphone, and basic mobile devices. Different landing page URLs can be used for different platforms, or Ajax can be used to display different advertisement variants on a page. CSS tables permit hybrid fixed+fluid layouts.
There are now many ways of validating and testing RWD designs, ranging from mobile site validators and mobile emulators to simultaneous testing tools like Adobe Edge Inspect. The Firefox browser and the Chrome console offer responsive design viewport resizing tools, as do third parties.
A site layout example that adapts to browser viewport width was first demonstrated by Cameron Adams in 2004. By 2008, a number of related terms such as "flexible", "liquid", "fluid", and "elastic" were being used to describe layouts. CSS3 media queries were almost ready for prime time in late 2008/early 2009. Ethan Marcotte coined the term responsive web design  (RWD)—and defined it to mean fluid grid/ flexible images/ media queries—in a May 2010 article in A List Apart. He described the theory and practice of responsive web design in his brief 2011 book titled Responsive Web Design. Responsive design was listed as #2 in Top Web Design Trends for 2012 by .net magazine after progressive enhancement at #1.
Forbes featured a piece, 'Why You Need To Prioritize Responsive Design Now'  where the importance was made clear that having a mobile version of your website isn’t enough anymore. Jody Resnick, President of Trighton Interactive, stated in his interview with Forbes, “Responsive websites simplify internet marketing and SEO. Instead of having to develop and manage content for multiple websites, businesses with responsive sites can take a unified approach to content management because they have only the one responsive site to manage. Resnick predicts, “As the internet transforms further into a platform of services and user interfaces that tie those services together, leveraging this technology in the future will allow companies to integrate a plethora of back-end services, such as Facebook, Twitter, Salesforce.com, and Amazon Web Services, and then present the integrated data back out the front-end iad layer on a responsive design so the application looks great on all devices without custom coding needed for each device or screen size."
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- W3C @media rule
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