Font Size and Browser Rounding
Specifying length (aka physical) units for font sizing can result in rounding when the browser converts the physical measurement to the nearest whole-number pixels on the display device. A similar conversion occurs when font sizes are specified as percentages or ems.
No such conversion needs to occur for fonts specified using whole-number pixels; further, advanced browsers may approximate font sizes explicitly declared with fractional pixels through anti-aliasing or when printing. (This would be another point for the pixel sizing advocates, for those who are counting!)
To get more consistent font size rounding results across browsers, use even-number values for percentages or ems.
For example, 76% or 0.76em produce more consistent results across browsers than 75% or 0.75em.