If you look at the source code on the Moustache site, it refers to a style sheet at http://d297h9he240fqh.cloudfront.net...oject_page.css
...and, in the style sheet, there's a 'body' statement -- body{font-family:"Helvetica Neue",Helvetica,Arial,sans-serif} -- that says, hey, folks, if you don't have Helvetica Neue, then show Helvetica; if you don't have Helvetica, then show Arial, and, if you don't have Arial, then show whatever your browser uses as a default sans-serif font.

Does this help? On my netbook, I see Arial, not having Helvetica Neue installed, and, being Windows, having Arial instead of Helvetica.

P.S. Four years later, am still put off Helvetica Neue -- we all had to use it at last job on a cellphone screen low-level design project, some idiot had specified it, and the company had to buy a zillion copies, when we could have used Arial Narrow. This wasn't highly graphical work, it was just blocking out where the different controls would go, and what would happen when keys were pressed. Aarghh!