Responsive design is a method of applying different sets of display rules  to elements of a website based on the size of the viewport availale.

These rules are applied via Cascading Style Sheets ( CSS ) and can affect the appearance, position, size or visibility of specific elements such as navigation, sidebars, pictures and the main content.

A single code base is created for the website rather than building a different website to be viewed on mobile phones, tablets and desktop computers.

Responsive design isn’t just a technical issue. A good responsive design requires design decisions to be taken to determine which elements to change as the viewport changes.

To see an example of responsive design in action, visit www.smashingmagazine.com with your web browser maximized. Now slowly resize the window until it is as small as it can go. The layout changes as you do this and as the viewport reduces in size, different parts of the style sheet are triggered and applied to the layout.

Opera is a web browser first publicly released by the Norwegian company Opera Software in 1996. It is available for many types of computer operating systems including Windows, Mac OS X and Linux.

Although not very common on desktop and laptop computers, it is very popular on mobile phones and PDAs as well as being the web browser for the Nintendo DS (a mobile games device) and the Nintendo Wii (a home games device).

Firefox is the name of a web browser created by the Mozilla Foundation. Web browsers are software programmes that we use to view web pages.

At present, Firefox is the second most popular web browser after Microsoft's Internet Explorer.

Like almost all web browsers, Firefox is free to download and use.

Camino is a web browser which is only available for Apple Mac. It is open source and is created by the Mozilla Foundation.