when you use caps in the stylesheet, for example Header, then the html tag div id="Header" will also be caps i'm sure it will still validate but it really should be lowercase to promote consistency accross the board, everyone has their way of course, like when you see people using PAGE.html or Page.html instead of page.html
XHTML requires all tag names and attributes to be lowercase. For CSS it is a personal preference, I personaly use camelCase in CSS. For the record -- class names and IDs ae case sensitive, so be careful there.