while pt = 1/72inch which is absolute.
so, if you really want to be EXACT same size on any monitor, you use pt over px.
Sorry to disagree but I believe that EM is the way to go to allow the user to change font size based on their own eyesight/preference. Using one of the fixed methods does not lend itself to WAI conformance.
I could be wrong but this site should have more information on the subject.
http://www.w3.org/WAI/
