h1.fo (not h1.xml):
<?xml version="1.0" encoding="utf-8"?>
<root>
   <header>O, my head!</header>
</root>

h2.fo:
<?xml version="1.0" encoding="utf-8"?>
<root>
   <xheader>O, my head!</xheader>
</root>

IE6 will display h1.fo in html and h2.fo in xml. The whole differences of the two files is h1.fo use "header" while h2.fo use "xheader" with additional "x".

Too nasty the bug was. It wasted me about one hours to find out why the *.fo (indeed it was an XML) was displayed in HTML.