@Font-face is the new hotness and last year was a huge year for non-standard fonts on the web, which has really opened up the possibilities for web designers. Typekit has been no small player in the increased use of @font-face use since it is easy to implement and has some great fonts (like Proxima Nova - the font
you are reading now I used to use on my personal site), but I had my reservations about embedding a font through JS.
My concerns were realized when I started using Typekit on this blog and every time the page loaded it initially loaded with my fallback Helvetica, then after a second or so would flash and substitute in Proxima Nova.
It took a while to resolve, but the issue was putting the Typekit <script> tag in the footer.
Even though putting JS at the bottom of the page is generally considered a best practice, Tumblr users who want to use Typekit need to put the necessary script tags in the <head> of the page.
Just wanted to share :)
Note: It should be said that there is still sometimes a very quick flash of "fallback" font, but putting the Typekit <script> in the head greatly decreases it.