I don't understand. Why would you use this instead of just putting the emoji Unicode character into the page and letting the device render it?
Differences between emoji between systems, perhaps?
Neat to see a CSS version of emoji, but wouldn't PNGs or otherwise be a much more efficient use of data to display emojis cross-platform, instead of a 74.5KB (unminified) CSS file being loaded to display a few emoji?
Edit: I suppose if you're using A LOT of emoji, at some point the tables turn and the CSS becomes more efficient...