I think also that "testObj.style.width" did not work because the "width" property was set in a CSS style sheet rather than in the inline style object for the DIV itself.
if you had
<div style="width:100px>My Div</div> it should work.
But as the width was in the CSS sheet, it cannot be accessed through the objects "style" property.
"offsetWidth" works because that is a dynamic property, which shows the final calculated width, taking account of CSS styles, and specific object styles, etyc.
There is also
"currentStyle" (or in Mozilla I thing it is getComputedStyle()) which can I think access the CSS style object
There is also (in IE only)
"runtimeStyle" which is a temporary style which you can apply to an object, overriding all other objects only whilst the page is being displayed.