This box-sizing polyfill is based on previous work done by Erik Arvidsson, which he published in 2002 on WebFX.
Since there were some edge/heavy usage cases where it broke I started pushing it further. And since the original was not aware of IE8 I also added feature-detection for box-sizing, to do nothing when detected positive.
During that I also borrowed dimension measuring techniques from Dean Edwards' IE7.js script.
Add the behavior/HTC after every box-sizing: border-box; that you assign:
box-sizing: border-box; *behavior: url(scripts/boxsizing.htc);
If you prefix the behavior property with a star, like seen above, it will only be seen by IE6 & IE7, not by IE8+ (it's a hack) which is better for the performance on those newer browsers.
The URL to the HTC file must be relative to your HTML(!) document, not relative to your CSS. That's why I'd advise you to use absolute paths like in the example.