Some browsers don’t support CSS as well as others. Some designs require CSS-hacks to assure browser compatibility. I think that the most used CSS hack must be the use of Conditional Comments, but the major disadvantage of Conditional Comments is that they require to change the HTML file. Sometimes it’s not possible or not convenient to change the HTML, for example when you already have a site with hundreds of pages and you don’t want to add the conditional comments to make use of the new design.