size: ee1 ee2 ee3 ee4 ee5
%: ee50 ee60 ee70 ee80 ee90 ee100 ee110 ee120 ee130 ee140 ee150 ee200 ee300 ee400 ee500
em: ee.5 ee.6 ee.7 ee.8 ee.9 ee1 ee1.1 ee1.2 ee1.3 ee1.4 ee1.5 ee2 ee3 ee4 ee5
Both in Firefox and IE size=3 has no effect. Also, in both, font-size=100% has no effect, as expected.
In Firefox 1em has no effect, as expected.
However, in IE 1em enhances the effect of the font size specified in the browser: large becomes even larger, small becomes even smaller. Accordingly, even .9em, while probably intended to decrease size, increases it if the size was already large.
Conclusion: use %, not em!
Option "ignore font sizes"Edit
The small/big tags work in IE even when the option "ignore font sizes" is on.