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The way Web sites are accessed by the users is evolving. A few years ago, the Internet was available only on desktop devices. Nowadays not only can Web sites be accessed from most mobile phones, but they are the only available way to access the Internet for a significant portion of the world's population.
Web developers must nowadays understand the specificity of mobile devices and design their Web sites so that they will be compatible with today's and tomorrow's world wide Web.
One URL for your unique content
There is only one Web. People access it using a wide range of devices: from phones with small screens to desktops with 1-meter screens. Moreover, not only do different users use different devices, but users use different devices at different moment.
Limitations of mobile devices: screen size, touch screens, bandwidth
Different engines, devices and users: making browser-agnostic HTML and CSS
Feature-detections rather than UA-sniffing