A px, em, % or pt is missing after the number, try
HTML Code:
font-size: 12px;
font-family: arial, sans-serif;
If you use em instead of px the font can grow along with the other fonts in the theme when someone chooses to increase the font size through his browser settings
HTML Code:
font-size: 1em;
font-family: arial, sans-serif;
Possible values Xem, X.Xem and X.XXem