I avoid using the defaults with the reset code below:
margin:0px; and padding:0px;
For example, what are the default margins for the headin
It varies between browsers. Check each browser's specific default stylesheets to tell.
For Google Chrome for example, it's 0.67em.
Do know that if you wish to aim for x-browser consistency, you'll have to use a CSS Reset.
The most common one being:
* { padding: 0; margin: 0; }
Although other (and probably better) exist.