f7723559_CSS_HTML_XHTML_Reference_Book__Description_and_notes
[Property/Values] margin-top, margin-right, margin-bottom, margin-left either a length, percentage, auto, or inherit [Description and notes] 
for setting the amount of space between only one side of an element's border and its parent element initial value: 0; not inherited; percentages refer to width of containing block; the values for margin-right and margin-left may be overridden if sum of width, margin-right, and margin-left are larger than parent element's containing block
CSS HTML XHTML Reference Book
Description and notes