I've been working hard recently to develop page templates for a site being developed in ASP.NET. At the outset of the project I decided the time was right to stop using table based page layouts, and move onto a fully accessible, CSS based design/layout. After playing about with floating DIV's to get them correctly positioned for each element of the page I found, however, that IE6 was rendering the DIV's a different size to Mozilla and other standards based browsers.
After doing some further research, I've found that what I had failed to use was a full and correct DOCTYPE. One of my main sources of information, css.maxdesign.com.au, has an article on the IE box model and Doctype modes. After reading it through, and then fixing the page to use the correct doctype, it all works fine!
Of course, I've still got the IE5 quirks to check out and work round, but how hard can that be ;-)
So, if you're developing a standards based site, make sure the DOCTYPE is correct!