What is the CSS to define the margins of the webpage?
It's possible to use: leftmargin="0" topmargin="0" marginwidth="0" marginheight="0"
within the <body> tag, but that doesn't produce XHTML 1.0 valid code.
You can set the margins of the <body>
Depending on the layout you're looking for you could also use different variations of <div>'s and css.
Check out glish's css layouts for some more detail.