The reality is you can use either perecents or ems to get the same affect,
but percents seem to be the preferred option (at least below 1em):
"A word of caution concerning IE. Be careful using ems. The most recent
versions of IE for Windows tend to flummox text with a font-size less than
1em ("0.5em", for instance). Percentages tend to work more predictably, and
(for who knows what reason) are usually more accurate (possibly rounding
errors?) than their em equivalents. Please note that this applies only to
the font-size (FontSize ) and line-height (LineHeight ) properties. "
http://css-discuss.incutio.com/?page=UsingEms
Russ
>
> Peter, you asked,
>
>> Do you feel the the pro's of using em's (for font sizes)
>> outweigh the cons?
>
> I'm sure there'll be some much more knowledgeable people than I on this
> subject, but I'll just make a couple of remarks.
>
> 1) Only IE on the PC (and I only learnt this recently, from John
> Allsopp) refuses to scale fonts given a pixel size. All other browsers
> (following the W3C spec, I believe) scale all fonts no matter what
> their units are. Will IE stay this way in Longhorn? Who knows. But
> relying on absolute sizes like this is relying on a bug, it seems to
> me.
>
> The point of scaleability is, as I'm sure you know, to allow for people
> whose sight is not the best to increase the size of everything so that
> they can read it.
>
> 2) If em measurements are used both for font sizes and to control the
> sizes of their containing elements, everything should scale nicely.
> However, because ems vary in 'real' size depending on their context, I
> can see that sometimes there are mismatches that are hard to overcome.
> Perhaps percentages would be more useful for those instances.
>
> Hugh :)
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