A realistic strategy today is to assume a typical desktop monitor by default, since not all popular desktop browsers implement media queries to a useful extent. Then you can add specific CSS sections for hardware with very different parameters, such as screens on mobile devices. Usually being both newer and more in need, mobile OSes and the browsers running on them typically support media queries better, so while not always ideal, this approach works fairly well in practice.
http://www.w3.org/TR/css3-mediaqueries/
A realistic strategy today is to assume a typical desktop monitor by default, since not all popular desktop browsers implement media queries to a useful extent. Then you can add specific CSS sections for hardware with very different parameters, such as screens on mobile devices. Usually being both newer and more in need, mobile OSes and the browsers running on them typically support media queries better, so while not always ideal, this approach works fairly well in practice.