> Let's be careful about propagating this bug description.  It is no longer a problem in IE6.

You're thinking of the 'font-size: medium'-not-being-equal-to-default-font-
size-bug, which is indeed gone in IE6, though only when in Standards Mode.

This is different, and only appears when the 'Text Size' option (on the 'View'
menu is set to something other than 'Medium'. IE then over-scales any text
set in ems relative to the default font size.

For a demonstration, try this:

    <p> Foo <span>Foo</span> </p>
    <p style="font-size: 1em;">
      Foo <span style="font-size: 1em">Foo</span>
    <p style="font-size: 100%;">
      Foo <span style="font-size: 1em">Foo</span>

Clearly everything here should be the same size. And it is when 'Text Size' is
'Medium'. Any other setting and the text set purely in ems is all wrong
(however the 1em inside the 100% is okay).

I find very commonly that users have Windows set to 'Large Fonts' (because the
default non-customisable icon font in Windows is so small I guess), and then
set IE to the 'Smaller' text size to compensate. In this case the over-scaling
problem can make the text on pure-em pages unreadably small.

Solution is either to declare a 100% font size in an ancestor element (eg.
body), and then use em inside that, or, my favoured solution, just use %