To display a website successfully on a range of devices, you need to create separate styles for at least these categories:
Mobile phones. There’s a wide range of screen sizes, but 320 pixels seems to be the most common among modern smartphones. However, the BlackBerry Torch screen is 360 pixels wide, and the Samsung Galaxy Tab is only 400 pixels wide in portrait orientation.
Tablets. Again, there’s a great deal of variety, but the iPad’s 768-pixel width is probably a good choice for the maximum limit. This set of rules applies to all devices wider than a mobile phone but no wider than 768 pixels.
Desktops. These rules apply to any device wider than 768 pixels.