Reply To: Implementing the box-sizing polyfill by Christian Schepp Schaefer in wordpress

[Alen]

You would need to upload the file to your theme folder. Then just include it in your CSS.

*{
-webkit-box-sizing: border-box;
-moz-box-sizing: border-box;
box-sizing: border-box;
*behavior: url(http://path/to/script/boxsizing.htc); }

Read here: https://github.com/Schepp/box-sizing-polyfill
And here: http://reference.sitepoint.com/css/behavior

(Code not tested)