Absolute positioning

Removing an element from normal flow and placing it against its containing block, so it no longer affects the size or position of anything else. It is the right tool for overlays and decorations that must not disturb the layout, and the wrong one for anything whose size depends on content, because the parent can no longer grow to fit it. The `position: relative` on the parent, which does nothing visible on its own, exists purely to become that containing block.