This describes the vertical offset for the bottom edge of the absolutely positioned element box from the bottom edge of the element's containing block. For relatively positioned boxes, the offsets are relative to where the box would appear normally in the document flow. Positive values are above the parent block's bottom edge and negative values are below.
Default offset in the regular layout of the page.
Negative values are allowed. Refers to a percentage of the height of the parent containing block. If the parent containing block does not have an explicit height, this value is interpreted like 'auto'.