Specify a Maximum Width for Em-Based Layouts
http://www.456bereastreet.com/archive/200803/specify_a_maximum_width_for_embased_layouts/

Johansson, Roger
456 Berea Street
2008

Abstract:

One technique that can easily make reading a site a lot more uncomfortable is using an elastic, or em-based, layout such as the one I use here (and talk about a bit more in detail in Fixed or fluid width? Elastic!) without specifying a maximum width in another unit.