DiapDealer published two days ago a script that converts
cm to
em directly within EPUB CSS stylesheets using Calibre.
More about it here
https://www.mobileread.com/forums/sho...6&postcount=30
It works both with condensed and "beautified" (classic) stylesheets. However, it does not convert values written in shorthand and leave them untouched.
Of course, still experimental.
Exemple with px in the screenshot for shorthand illustration. Number four (lowest one) should be OK however.