I had a brief issue with a quickly thrown together website. Specifically the margin:auto; centering was not working in IE8 and jQuery scripts were not behaving. I found out that this was related to the lack of a DTD e.g.
<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Strict//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd”>
More explanation of doctypes is here.