Stacking Order
One of the side effects of positioning is that elements
can overlap each other. By default, elements stack in the order in
which they appear in the document, with later elements rendering on top
of preceding elements in the source. You can change the stacking
order for an element by setting the z-index
property. You can picture the
direction of the z-axis as a line that runs from your nose through this
page and out the other side.
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