The W3C has a comprehensive default CSS document, which basically just codifies the default practices of most existing visual browsers. It's a nice base to work off of, especially if you're going to make extensive changes to the display of common elements.
One change I would make for most web projects would be to exclude FORM from the big list of block elements at the beginning, and perhaps use the following instead:
form { display : inline; }
This one little change would help eliminate most of the "the <form> tag always inserts extra space!" problems that web designers using <table> layouts have struggled with for years.
In Defense of Marriage