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Stacking is a three-dimensional conceptualization of web pages, where an imaginary z-axis runs perpendicular to the screen. On the xy plane elements can overlap and the browser needs to figure out which one should be on top. There are three types of elements to consider when it comes to the default stacking order: the root, non-positioned, and positioned elements. The root element goes at the bottom of this imaginary stack, then the non-positioned elements, and finally the positioned elements.