For relatively positioned elements, the top or bottom property specifies the vertical offset from the normal position and the left or right property specifies the horizontal offset.

For absolutely positioned elements, the top, right, bottom, and left properties specify offsets from the edge of the element’s containing block (what the element is positioned relative to). The margin of the element is then positioned inside these offsets.

Most of the time, absolutely positioned elements have auto values of height and width computed to fit the contents of the element. However, non-replaced absolutely positioned elements can be made to fill the available space by specifying (as other than auto) both top and bottom and leaving height unspecified (that is, auto). Likewise for left, right, and width.