A further addition, what is happening is in quirks mode, some selectors will fail, some will not. When in quirks mode, it's left up to the browser to decide what's "valid" and what's not, so it's going to vary from browser to browser.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
A point to consider is why you are using an XHTML doctype. If your page is plain HTML, this not only misrepresents the document type, it makes validating a little more difficult because some elements are not supported by XHTML strict.
Choosing the best doctype for your site [webmasterworld.com]
Why most of us should NOT use XHTML [webmasterworld.com]
[edited by: encyclo at 11:05 pm (utc) on Mar. 1, 2009]
[edit reason] fixed link [/edit]