Eric Meyer made a test of how browsers handle an increasing size of text elements (nesting a load of
<b> elements with a
font-size of 1.04em in an element with a
font-size of 10px), and enters a philosophical discussion of whether browsers should display the actually used
font-size (that is, the rounded one they use for displaying) or the one they carry with them (unrounded).
I personally think Safari and Opera’s handling is the most correct (the computed value must be the one that the user can actually experience), but I have nothing to back that up with.