I had a similar bloody nightmare with a site I built recently for a local business.

CSS is a great way to build sites. Much better than the old way of filling the code with tables, but there's a bug of some kind that means you have to "clear" elements in some cases where you use a lot of "float" positioning, otherwise it screws up the display of background colours.

The biggest problem has always been that different browsers render HTML and CSS differently. IMHO, Internet Explorer is the biggest b*stard on that count.