Safari 1.3 Opera 7.54 Opera 8 Firefox 1.0.4 Netscape 7.1 IE 6 Yours Method
586px 0px 596px 586px 586px undefined document.defaultView.getComputedStyle(elementNode,"").getPropertyValue("width")
undefined 0px 596px 586px 586px undefined window.getComputedStyle(elementNode,"").getPropertyValue("width");
undefined undefined undefined undefined undefined auto elementNode.currentStyle.width
  0 0 undefined undefined 0 elementNode.style.pixelWidth
596 596 596 596 596 596 elementNode.offsetWidth
592 592 592 592 0 0 elementNode.clientWidth
596 596 596 596 596 596 elementNode.scrollWidth
             
Safari 1.3 Opera 7.54 Opera 8 Firefox 1.0.4 Netscape 7.1 IE 6 Yours Method
341 341 341 349 349 335 getOffsetTop
349 349 349 349 349 343 getOffsetTop + document.body.offsetTop
undefined CSS1Compat CSS1Compat CSS1Compat   CSS1Compat document.compatMode
             
undefined   3px 3px 3px undefined window.getComputedStyle(elementNode,"").getPropertyValue("padding-left");
undefined   undefined undefined undefined 3px elementNode.currentStyle.paddingLeft

This is the Test paragraph.
It's dimension are set through CSS.
#Container {
width: 600px;
padding: 3px;
background-color:#FFCC66;
margin: 1em 0;
}
#Test {
margin: 2px;
padding: 3px;
border: 2px solid #bbb;
background-color:#FFCCCC;
font-family: "Courier New", Courier, mono;
}

http://www.georgehernandez.com/xWebs/CSS/IEDimensions.htm