It took me a bit to figure out what exactly it was doing on your site, especially with it turning purple. Having a three step sample use-case guide on your homepage would be helpful.
Also, rather than specifically tell it what font to fallback to, I'd love to be able to just choose an operating system and have it show me what font would have been used by a user on a default installation of that OS.
It would be helpful if you just had a demo video (short video), showing what the site does. It took me a few seconds to even understand the concept fully. In those seconds, you could lose many potential users.
Couldn't quite figure out how to use the tool, but it did manage to create a pretty rad 3D version of my site (using the "Both" setting and varying font sizes on each). I'll probably have another go when I've got less homework.
I'm sorry if this is not the world's best HN comment, but I really enjoy the aesthetics of this site. The colors and font choices are fantastic, at least to me. Thumbs up.
This comment essentially got me to click the link, and I agree (with the exception of the animated-on-hover bookmarklet... that's nasty). I was curious, but not overly.
speaking of magical unicorns and javascript rainbows... (might take a moment for the first run to work)
Also, rather than specifically tell it what font to fallback to, I'd love to be able to just choose an operating system and have it show me what font would have been used by a user on a default installation of that OS.