The width of the left border of an element in pixels. It includes the width of the vertical scrollbar if the text direction of the element is right–to–left and if there is an overflow causing a left vertical scrollbar to be rendered.
clientLeft does not include the left margin or the left padding.
clientLeft is read-only.
var left = element.clientLeft;
When layout.scrollbar.side preference is set to 1 or to 3 and when the text-direction is set to RTL, then the vertical scrollbar is positioned on the left and this impacts the way clientLeft is computed.
clientLeft was first introduced in the MS IE DHTML object model.
The position of the vertical scrollbar in right–to–left text direction set on the element will depend on the layout.scrollbar.side preference