I see you have support for Typekit as well. Very cool.
This is pretty damn awesome, I have to admit. One of the big pains of working with web font services like Typekit is that you have to select a set of fonts beforehand, save the kit, change your CSS, and only THEN can you see a change. At best you can keep a handful of browser tabs open without refreshing them so that you can flip between them to compare. But this makes everything so much easier.
Also hidden in Firefox 25 for me. I'm constrained for vertical space on my netbook, which did make me think you could display at least a couple more typefaces at a time in the left hand menu if it wasn't for vertical space used up by the menu bar at the top, and the line with "$number typefaces"
When you change the style of an element, like a paragraph, it works like CSS and applies the change to all elements of that type. To add specificity, you have to give an element a class.
There are some important things missing, as far as I can tell though, like ... how do I make a list?