Gets/sets the width of the frame's margin in pixels.
The sWidth string gives the width of the frame's margin in pixels.
Note that the data type for the value of the marginWidth is a string. This is because the value may be specified as either a number of pixels (e.g., "2") or a percentage, in which case the percent sign is included. A value of "0" means no margin at all.
DOM Level 2 -- HTMLIFrameElement