CSS and case-sensitivity

One thing that bothered me when I was learning css a few years ago is that
most of the tutorials/guides out there (even the ones from some "experts")
ignore case-sensitivity issues.

They'll usually say something like: for element <p></p>, write a selector P
{}. Usually the reason given is so the capital letters will "stand-out" from
the rest of the text. This was the same reason given for capitalizing html
tags 5 years ago... Now that xhtml is case-sensitive lower-case, shouldn't
css be too?

You can use css on any xml file and since xml is case-sensitive, what's to
differentiate <thisTag /> from <THIStag />... Shouldn't the css selectors be
thisTag and THIStag?

Maybe they are and the browsers just don't care yet. Will someone please
shed light on this for me? Thanks in advance.

James Craig