It works, scroll down... it loads the preview underneath the post you are composing. Not very intuitive, I know. Maybe I'll add a loading GIF and scroll you down when it's ready.
Something most will not care about, but I think Z mentioned it before.
Gzip compression and expiration headers are being set appriorately for all HTML, CSS, JS, and images. So especially if you are viewing the site on your phone not on wifi, it will use less data for your phone plan.