If a table mixes fonts, you could choose for the em-size:
default for document
minimum
maximum
average
median
most commonly used font
I'm starting to lean toward 'maximum' as giving a more likely method
of achieving this goal:
> What matters here is for the author to have
> a way to specify the width of the table in units that are meaningful
> in the ultimate consumer's context such that the information intended
> to be conveyed by spatial relationships is preserved.