A simple utility to execute a callback when a webfont loads.  Demo

This uses the CSS Font Loading Module when available (currently in Chrome 35+ and Opera 22+). When that isn’t available, it uses a very similar approach to the one used in the TypeKit Web Font Loader (which is currently 7.1KB GZIP).

Basically, it creates an element with a font stack including the web font and a default serif/sans-serif typeface. It then uses a test string and measures the dimensions of the element at a certain interval. When the dimensions are different than the default fallback fonts, the font is considered to have loaded successfully.

If you’d like a full polyfill for the CSS Font Loading Module, follow along with Bram Stein’s Font Loader. I believe the specification has changed since he launched this polyfill, but he’s working on an updated version.