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Ayaz is again correct here - responsive design lives entirely on the front-end and utilizes CSS3. Essentially, it adjusts/resizes content depending on the viewport/browser, so as screen sizes get smaller, the content layout adjusts accordingly, certain elements disappear completely, etc - but it's the same URL/source code, so there's no need to detect the user agent and redirect to a separate mobile site. Requires no particular server technology or environment, but you may find your content management system, if you're running one, makes it difficult to implement - though most currently-updated CMSs, like WordPress, are starting to integrate it.