Quote:
A key concept of EPUB is that content presentation should adapt to the User rather than the User having to adapt to a particular presentation of content.
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(Taken from http://www.idpf.org/epub/30/spec/epub30-overview.html )
I don't know if all the readers fully support it but it's quite possible for ePub authors to specify line height in percent, just like (almost) everything else. Not that it automatically solves all the potential scaling problems etc., but when you really really need to mess with line height, that should be the way to go.