In an ideal world, you only need one set of CSS style sheets for your website, and those styles will work with every browser currently being used. This, as every webmaster soon finds out when he/she uses CSS, is a pipe dream. The modern browsers all have uneven levels of implementation of the CSS standards. As though this isn't bad enough, their implementations are often buggy - and they don't share the same bugs! And when you have solved that tricky bit, you find that your site has certain visitors...
