Normally, you would not need to specify this property. It allows the document to have some clear space at the bottom so, for example, the content could be made to scroll completely off the screen.
The distance is measured in pixels and can range from zero (which is the default) to any reasonably sensible value.
If a page is being created dynamically with document.write() methods, and a script error occurs, the margin is not appended. It appears to be added as a property of the body closure.
This corresponds to the style.marginBottom property and the margin-bottom attribute that is defined in a style sheet.
Even when this value is set to zero, the Macintosh vesrion of MSIE has a noticeable margin at the bottom.