HTML and CSS Reference
<length> | <percentage> | auto
Positioned elements (that is, elements for which the value of position is something other
than static )
Refer to the height of the containing block
For relatively positioned elements, see note; for static elements, auto ; for length values, the
corresponding absolute length; for percentage values, the declared value; otherwise, auto
Defines the offset between the top outer margin edge of a positioned element and the top edge
of its containing block.
For relatively positioned elements, if both top and bottom are auto , their computed values
are both 0 ; if one of them is auto , it becomes the negative of the other; if neither is auto ,
bottom becomes the negative of the value of top .