I really like the developer toolbar in IE8 although I'm still getting familiar with it. I am in the process of putting the finishing touches to a new site for a client and was validating the CSS and the XHTML. The CSS passed on the first try, but the markup took a while. Initially it barked because of an ampersand in the footer credits, but changing this to the corresponding "&" special character entity number did the trick.
Then I got the following error: "No Character encoding declared at document level..." I am using XHTML 1.0 Transitional:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
After much ado, the solution was to add the following to the head section in the master page:
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />