Created attachment 24866 [details] Example MOBI document. Steps to reproduce: `for s in 11 22 33 44; do mutool draw -S $s -o test-${s}.png test.mobi 1; done`. You'll notice that the font size of the main text changes while the font size of the chapter and initial cap are fixed.
commit 4ae522a299df7e18f155ae319c7756298d69c490 Author: Tor Andersson <tor.andersson@artifex.com> Date: Thu Sep 21 15:36:17 2023 +0200 Bug 707183: Use relative MOBI font sizes. Scale them with the base 'em' size instead of using specific point sizes.