The main body font in ace.css only specifies "Open Sans" without any fallbacks. Further down in the file, you can sometimes see alternative fonts being specified, e.g. for headlines. These fallbacks should also be specified for the body, otherwise most browsers will default to a font with serifs.

This can be problematic in particular on unstable mobile connections where the font may not load at all. As a result, the entire issue tracker will use Times New Roman or a similar font on most browsers.