While everyone has their browser of choice (IE and Firefox by far are
the most popular), it is important for web designers to test their
creations in multiple browsers. A site that displays a certain way in
IE may not display the same way in Firefox, especially when it comes
to CSS. Be sure to check your sites in at least these two browsers to
make sure they look the same. Remember not everyone uses the browser
you do, people do use others such as Opera, Konquerer, Netscape, and
even Lynx.

Even better, check your HTML and CSS against the W3C validators at
their site: [http://www.w3c.org]
This should pratically assure all browsers will interpret them correctly.