How does it work?
This code works on the simple principle that each character appears differently in different fonts. So different fonts will take different width and height for the same string of characters of same font-size.
We try to create a string with a specified font-face. If the font-face is not available, it takes up the font-face of the parent element. We then compare the width of the string with the specified font-face and width of the string with the font-face of the parent element, if they are different, then the font exists, otherwise not.