www.w3.org/TR/CSS21/visudet.html#abs-non-replaced-width
If all three of 'left', 'width', and 'right' are 'auto': First set any 'auto' values for 'margin-left' and 'margin-right' to 0. Then, if the 'direction' property of the element establishing the static-position containing block is 'ltr' set 'left' to the static position and apply rule number three below; otherwise, set 'right' to the static position and apply rule number one below.