Positioning

Four positioning schemes:

Value	Behaviour
static	Normal flow (default).
relative	Offset from its static position; keeps space in flow.
absolute	Removed from flow; positioned relative to the nearest positioned ancestor.
fixed	Removed from flow; positioned relative to the viewport (stays on scroll).
sticky	Behaves like relative until a scroll threshold, then becomes fixed.

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Example: Sticky header

header { position:sticky; top:0; background:#fff; z-index:10; }