Selection.anchorNode read-only property returns the
Node in which the selection begins.
A user may make a selection from left to right (in document order) or right to left (reverse of document order). The anchor is where the user began the selection. This can be visualized by holding the Shift key and pressing the arrow keys on your keyboard. The selection's anchor does not move, but the selection's focus, the other end of the selection, does move.
node = sel.anchorNode
The definition of 'Selection.anchorNode' in that specification.
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Selection, the interface it belongs to.