I had the same issue, using Firefox 33 on Windows 8.1. It looks like switching to Chrome fixed the demo. Would be nice to have some sort of notice that certain browsers aren't supported (if that's the case).
Also, fonts are rendering poorly in Firefox and Chrome [0]. Bumping the font-weight up to at least 400 makes the text much more legible.
Also, fonts are rendering poorly in Firefox and Chrome [0]. Bumping the font-weight up to at least 400 makes the text much more legible.
[0]: http://imgur.com/2wVXvUr