CSS absolute positioning

CSS absolute positioning positions a generated box with respect to its CSS containing block, removing it from the ordinary flow that places its siblings. The declaration position: absolute selects this positioning scheme; offsets and size rules then determine its geometry. Its siblings do not reserve the space that it would otherwise have occupied.^[ref]

A simple example uses a positioned block ancestor with content width $300$ CSS pixels, twenty-pixel padding on both sides and content-box sizing. In the ordinary CSS2.2 case, this ancestor's padding edges establish a $340$-pixel-wide CSS containing block for an absolutely positioned descendant, provided no nearer qualifying ancestor intervenes.^[ref]

Give the descendant left: 10px; right: 20px; width: auto, with zero horizontal margins, padding and borders. The horizontal constraint becomesso its used content width is $310$ pixels. Alternatively, width: 50%; left: 10px; right: auto gives a width of $170$ pixels under the same assumptions. Offsets and percentages therefore refer to the selected containing geometry, not automatically to the ancestor's content width.^[ref]

Absolute positioning does not remove the DOM element from the DOM tree; it changes layout participation. It can allow boxes to overlap, but does not by itself specify clipping, permanent attachment to a screen position or every painting-order relationship. Use CSS overflow and the applicable positioning rules when diagnosing those separate effects.