font: 15px normal 'Open Sans', Trebuchet, Arial, sans-serif;
In this line, the -normal- part gets parsed as one of the font names (See http://www.dafont.com/normal.font).
There isn't a comma between -normal- and -'Open Sans'-, so the parser is wrong.
But the editor is wrong as well because -normal- should be in front of the font-size.
If you have the normal font installed you can see this on (see attached screenshot):
http://html5-demos.appspot.com/static/filesystem/filer.js/demos/css/app.css on line 33
Created attachment 151090 [details]
Before and after
Created attachment 151619 [details]