This is an amazing idea, and I will be implementing this today. Great job. I haven't seen any rendering problems across browsers either.
The only change I would make is to remove the "Share on Twitter" text that is included with each large button on the website example. It's pretty much implied for the end user once they see the colors of the buttons and the icons.
For example, I shortened "Share on Twitter" to just "Twitter".
Along this point, I feel the 'medium' preset's removal of the logos is odd as it's not typically done. I'd rather see the medium preset be '[logo] Twitter' instead.
The only change I would make is to remove the "Share on Twitter" text that is included with each large button on the website example. It's pretty much implied for the end user once they see the colors of the buttons and the icons.
For example, I shortened "Share on Twitter" to just "Twitter".