Pango 1.50 introduced the possibility to use a percentage in the size setting, e.g.
<span size="120%">will make the text 20% larger. Great.
What I find missing is the possibility to use a percentage in the rise setting. For example, an imitation of the TeX logo could be
T<span rise="-10240" size="70%">E</span>X. When reproduced at different font sizes, this leads to unwanted results.