While comparing a few pages in a new web site project, I noticed some pages don't use the DOCTYPE tag. It took me a while, but I realized that without the DOCTYPE tag, certain CSS rules will not work.
margin: 0 auto;
any :hover or other pseudo classes
etc.
This may come up if you testing some CSS and your rules don't work...make sure there is a DOCTYPE tag on the html page at the very top:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
