The numerical properties of a given text format can be expressed relative to a base value. You can establish base values for calculating relative sizes of properties with units of em, en, or percent. These base values must be specified with one of the following fixed units:
To specify a base size for text formats:
[BaseValues] ; BodyBaseHeight = size to use as basis for calculating relative ; values for font size, line height, and all values using units ; of em or ex. If not set, 10pt is used. BodyBaseHeight=10pt
To specify a base size of the containing element (usually body) for margins, padding, and indentation:
[BaseValues] ; BodyBaseWidth = width to use as basis for calculating percent ; values for margins, padding, and text-indent. If not set, 6in ; is used. BodyBaseWidth=6in
Percent values are tricky; you might want to avoid their use unless you are very familiar with advanced CSS.