r.p. ancestor
static parent: width:50%;
a.p. child: width:50%; right:0; top:0;
<div id="rp_ancestor"> ...<div id="static_parent"> ...<div id="ap_child"> ...</div></div></div>(CSS2.1 10.1).Definition of 'containing block': ... 4. If the element has 'position: absolute', the containing block is established by the nearest ancestor with a 'position' of 'absolute', 'relative' or 'fixed'...
(CSS2.1 10.2).Note: For absolutely positioned elements whose containing block is based on a block-level element, the percentage is calculated with respect to the width of the padding box of that element. This is a change from CSS1, where the percentage width was always calculated with respect to the content box of the parent element ...