Created attachment 255717 [details]
Example program showing too-small rise with a large font

Current pango uses an absolute vertical displacement to implement the
<sup> and <sub> tags in pango markup, (such as rise="5000" for
<sup). This value is only close to correct for a limited range of font
sizes. For very small fonts, the superscript glyphs will be displaced
too far, and for large font sizes, the displacement will be far too
small.

If there were a new attribute available, such as PANGO_ATTR_RELATIVE_RISE,
then it would be straightforward to use that for <sup>/<sub>.

There seems to be some consensus around using 33% for the displacement
in cases like this, though some software uses smaller values:

http://en.wikipedia.org/wiki/Subscript_and_superscript#Desktop_publishing

Of course, that page also demonstrates that a single relative value is
not correct in all cases, (a suffix such as "th" should have a rise
that's smaller than a numerical exponent, for example). So allowing
for things like rise="25%" would also give the user some good
flexibility to control this.