The Typekit kit editor provides two versions of the embed code, the default version blocks rendering and the asynchronous version which can produce a flash of unstyled content (FOUC).
In the case of Typekit, I prefer the FOUC over slowing down rendering. The asynchronous code also offers the advantage of your site staying online should Typekit go down.
I decided to spend a little time optimising the asynchronous code Typekit provides.