So that if the [TESTING-FONT] exists, then the 2 span's offsetWidth should not be the same. If they have the same width, that means their font are the same -- monospace, and also means the [TESTING-FONT] is not exist.
Use this tool on blogs or webpages to check of the font is installed on the computer!
It works on all popular browser: Firefox, Chrome, and even IE (6 and above)!
Installation: Download the font-checker.js file and place it in the same folder of your webpage, then insert this between head or inside your post if you are blogging: