position of baseline relative to em square

Hi font experts,

CSS says that one must center text inside its inline box. The spec is a
bit vague, but I am wondering whether such centering should be based on
the em square or the bounding box.

If it is to be the bounding box, then it can be implemented quite easily
on Windows, since tmAscent gives you the max ascent, and so you can
specify the Y coordinate based on the line-height and this max ascent.

However, if CSS wants us to center in terms of the em square, we would
need to know where the baseline lies in the em square.

As far as I can tell, neither the Windows APIs nor the TrueType file
give us this info. Is this true?

Thanks,

Erik van der Poel
Netscape