[Brett Merkey]

@Phssthpok,
If Calibre routinely alters CSS font-size keywords during conversion, this could explain both the readability issues in the conversion result and the fact, as you report, that only some books have the problem.

I did not notice this Calibre behavior before you brought the issue up because I always convert those CSS keywords to ems in the epub *before* conversion to AZW3. I recommend avoiding this Calibre issue by using regex to blast away those CSS keywords in the epub. I posted my own workflow solution in the Editor section of this forum. This will allow you to maintain a more fluid control of typography. The translation of keywords to point units for a screen medium does not make much sense and should be avoided.