HTML and CSS Reference
However, a browser that misinterprets the CSS box model, such as Internet Explorer 5,
would include the border and padding in the measurement, so it would subtract these
values to produce the rendered region:
With such a vast difference in measurement, layout variations are quite noticeable:
IE 5.5 needs fix
IE 6+ no fix needed
If you have a very old browser, try it yourself at http://htmlref.com/ch4/boxmodelhack.html.