The only problem with this is that, for historical reasons (and lack of caring?), the default stylesheets for unstyled HTML in the leading browsers are terrible. Best to apply a simple classless style sheet once then forget about it.
There's not that much that is objectively bad about the default styles. About the only thing that I can really put my finger on is equal top/bottom margins on headings. Possibly default line height, too. I wonder if one of the browsers will (or already has?) ever change that.