What is the key difference between absolute and relative positioning in CSS?

The distinction between absolute and relative positioning in CSS is indeed best captured by the concept that absolute positioning relies on the nearest positioned ancestor. When an element is given absolute positioning, it is removed from the normal document flow and is instead positioned relative to the closest ancestor element that has a position value other than static (i.e., positioned with relative, absolute, or fixed). This allows for precise placement of the element regardless of the surrounding content, enabling you to layer elements or create complex layouts.

Conversely, relative positioning allows you to move an element in relation to its original position in the document flow, meaning that it still occupies space in the layout, even though it appears shifted visually. Because the element is not removed from the flow, the layout around it remains unaffected in terms of space.

In contrast, the other choices do not accurately reflect the principles of CSS positioning. Absolute positioning is not determined solely by the viewport, nor does relative positioning specifically relate to the next sibling element. Additionally, while relative positioning does allow for visual adjustment without disrupting the document flow, this aspect is not the key defining difference to compare it with absolute positioning.