<p>When setting the size of fonts in CSS, should I be using a percent value (<code>%</code>) or <code>em</code>? Can you explain the advantage?</p>

From
http://archivist.incutio.com/viewlist/css-discuss/1408
%: Some browsers doesn't handle
  percent for font-size but interprets
  150%  as 150px. (Some NN4 versions,
  for instance.) IE also has problems
  with  percent on nested elements. It
  seems IE uses percent relative to
  viewport  instead of relative to
  parent element. Yet another problem
  (though correct  according to the W3C
  specs), in Moz/Ns6, you can't use
  percent relative to  elements with no
  specified height/width.
em: Sometimes browsers use the wrong
  reference size, but of the relative 
  units it's the one with least
  problems. You might find it
  interpreted as px  sometimes though.
pt: Differs greatly between
  resolutions, and should not be used
  for  display. It's quite safe for
  print use though.
px: The only reliable absolute unit on
  screen. It might be wrongly 
  interpreted in print though, as one
  point usually consist of several 
  pixels, and thus everything becomes
  ridiculously small.