One of my dreams is that every browser in the world renders CSS and XHTML the same way as Firefox does.
But this is just a dream, all the elements ususally have different parameters in different browsers (worst ones are the IEs), as a result it’s a hell for us.
As a result, even a totally valid CSS/mark-up website will look different under different browsers.
So what could we do? Simply, look every bit of your website under all the browsers, and see what’s different. After you see what’s different, redefine the CSS rules, so that they all match.
How browsers SHOULD render CSS: http://www.w3.org/TR/xhtml2/xhtml2-style.html