CSS FAQ
Questions about percentages
It is relative to the parent element's font-size.
For example, if the style sheet says:
H1 {font-size: 20pt}
SUP {font-size: 80%}
a <SUP> inside an <H1> will have a font-size of 80% times 20pt, or 16pt.
There was some confusion over this issue, since Microsoft's Internet Explorer version 3 had an erroneous implementation of percentage values in SUP elements: the percentages was computed over the default size of the SUP, not over the inherited size. [Information provided by Braden N. McDaniel <braden@shadow.net>]