Comment by cxr

The "online site"/"web thingy" linked in another comment is

1. like 300 lines of code <https://github.com/innocenat/kindle-epub-fix/blob/master/scr...>

2. runs entirely in the browser and isn't using Calibre scripts or anything else

3. could pretty easily be modified into a downloadable program that runs from both the command-line or something that you can open in your browser