Nowadays, the common phenomena is to develop a website for a browser and then corresponding apps for Android phones, iPhone, tablets and so on.

Since all the platforms come with a browser, why aren't companies using CSS to accommodate them? Surely we can detect from the request which browser was used and from which platform the request came. Reading those values, why don't we just implement the corresponding CSS for different platforms. Like we do for IE, Chrome and Safari. This way we can use the platforms' browser capabilities and don't need to develop subsequent apps for a platform.