Add a comment about only inheriting missing values.
Add a comment about only inheriting missing values.

andrewleech/WeasyPrint,prepare/TestWeasyPrint,marclaporte/WeasyPrint,jasco/WeasyPrint,Kozea/WeasyPrint,jasco/WeasyPrint,Kozea/WeasyPrint,andrewleech/WeasyPrint,prepare/TestWeasyPrint,marclaporte/WeasyPrint