In modern browsers you can target <em>ranges</em> with <a href="http://www.w3.org/TR/css3-mediaqueries/">Media Queries</a>. They're awesome. Assuming you don't care about IE – although IE9 promises support for them. Conditional comments can be used to send a lowest-common-denominator-resolution to IE, or JavaScript can be used to dynamically load stylesheets. Loading CSS through JavaScript dynamically is, in my opinion, an <em>awful</em> idea. A web designer named Jon Hicks <a href="http://hicksdesign.co.uk/journal/finally-a-fluid-hicksdesign">recently redesigned</a> his website using them, go there and change the size of your browser window to see an example of how they work. There are layouts from 1- to 4-column, depending on your current maximum viewport width.