Bug rendering vertical-align:top for a superscript

If you put a superscript or a subscript, lines get extra vertical 
spacing. If you impose vertical-align: top (resp. bottom), this effect 
is corrected: superscripts or subscripts stay in their line, no extra 
spacing appears. This is at least how Firefox renders it.

This is not rendered by Amaya (11.3 pre under Ubuntu 9.10). The clause 
vertical-align: top (resp. bottom) makes no difference in Amaya. You get 
extra vertical spacing anyway.

Example attached. First superscript is a plain <sup>, the two others 
have vertical-align: top. First subscript is a plain <sub>, the next one 
has vertical-align: bottom. Amaya makes no difference, but Firefox does.

(I do not mean that my superscripts and subscripts are nice when inside 
the normal line height in this example. This is only to illustrate the 
problem. When actually using this in my pages, I impose a smaller font 
on them, like I did for the 21th century in the example.)