Comment 15 for bug 728408

It is not a dumb question. It is often asked and very hard to answer. You need to compare the order of precedence of the requested fonts against those that are provided by your browser via the OS/font render. This can be difficult because you need to know what tools can display fonts and know how to read their names when comparing them to the fonts requested. Lp uses two font-families for normal (roman) and monospaced text:
    normal: 'UbuntuBeta Regular', Ubuntu, 'Bitstream Vera Sans', 'DejaVu Sans', Tahoma, sans-serif;
    monospaced: 'UbuntuBeta Mono', 'Ubuntu Mono', monospace;
^ The rule is favour the testing ubuntu font, the stable ubuntu font,
  older sans-serif fonts used by free systems, older fonts used on proprietary systems.

If you are testing Ubuntu fonts, you are seeing UbuntuBeta (that change often). If you are running Ubuntu 10.10, you are seeing Ubuntu, Older Ubuntu an free systems will probably match DejaVu Sans. Windows and MacOS will choose Tahoma.