Retrieves the offset of the starting boundary point relative to the startContainer in the current range.
the offset value.
If the boundary point offset is within a container that is not a character data node, the offset is essentially a count based on where it falls between child nodes. For example, the offset is 0 if it falls before the first child and 1 if it is between the first and second child. If the offset is within a character data node container, the value represents 16-bit unit positions.