I looked at Launchpad bug #523189 today. This bug cannot be fixed until Launchpad abandoned YUI-Font rules — percentages have unpredictable results when pages are constructed from fragment. Canonical own Web guidelines specify the use of pixels.
Launchpad uses YUI-Font to normaize font sizes across browser and OSes. It stipulates that all font sizes must be expressed in specific precentages to get the expected point size. This is difficult to use because browsers know that percentages are relative; nesting classes have surprising results.
The following text uses nesting with the same percentages that Launchpad uses.
Launchpd pages are constructed from many html fragments. The fragments are intended to be usable in many places on a page and display the the same way. Launchpad cannot use percentages for font-size.