In many browsers, you can use the max-width CSS attribute to set a maximum width for a given portion of a webpage. Among other things, this allows you to constrain a the width of a large amount of text to a given width. It’s a well known fact that people are much more comfortable with line-lengths of around sixty characters than longer ones. Therefore setting a maximum width for a column of text allows you to keep line-lengths within reasonable limits.
However, as with many things, this particular property is one that This article explains how you can use expressions to emulate max-width, and how to use it without breaking other browsers. While the expression idea is pretty good, I often find that I have to use it to make up for CSS properties that IE doesn’t support. If MS hadn’t spent their time doing this “cool” feature, and perhaps spent a little longer on security and CSS… but then, why would you want to do that?