Nikolai, 

I'm using FOP, so no "baseline-shift" ...
"vertical-align" is supported and the outcome is must of the time ok.

Greetings,
Dineke van den Bos

Dineke,

> <fo:inline vertical-align="sub">

your XSL is OK except for the name of the property. 
It should be "baseline-shift" instead of "vertical-align":

<fo:inline baseline-shift="sub">....
<fo:inline baseline-shift="super">....

Whether your XSL-FO formatter supports this is another issue...

Best regards,
Nikolai Grigoriev
RenderX

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list