CSS FAQ
Questions about 'auto'
In vertical margins, 'auto' is always equal to 0.
In horizontal margins, 'auto' is only equal to 0 if the 'width' property is also 'auto'. Here are three examples, assume that there is a <P> that is a child of <BODY>:
BODY {width: 30em}
P {width: auto; margin-left: auto; margin-right: auto}
Since the 'width' property is 'auto', the 'auto' values of the two margins will be ignored. The result is a P that is 30em wide, with no margins.
BODY {width: 30em}
P {width: 20em; margin-left: auto; margin-right: auto}
The P will be 20em wide and the remaining 10em will be divided between the two margins. Paragraphs will be indented 5em at both sides.
BODY {width: 30em}
P {width: 20em; margin-left: 2em; margin-right: auto}
Paragraphs are 20em wide and are idented 2em on the left side. Since the total width available is 30em, that means the right margin will be 8em.
Note that the default value of 'width' is 'auto', so setting one or both margins to 'auto' is only useful if you set the width to something other than 'auto' at the same time.