In my opinion em would be more predictable. With the current configuration, users of this package would only be able to adjust the font size by setting font-siz

In my opinion em would be more predictable. With the current configuration, users of this package would only be able to adjust the font size by setting font-size on the html element, but not on the body element or any container elements of the main element, or even the main element itself. That is pretty confusing to me, I think it is a pretty typical expectation that making the font size of a container bigger or smaller should always make everything inside proportionately bigger or smaller.