That... was incredibly helpful. Lightbulb moment. Thank you.
I think i understand how this is working, and why changing the max-height before was making things overlap in my version. Your code sets the max height at the container level, not the img level. In your epub3 solution, in order set the image height to, say, 2em, I have to add that to the css for ".autoImg img", not just ".autoImg".
What I'm still a bit flummoxed by is why this wouldn't work in epub2! I'm not seeing any properties that, individually, aren't recognized by that format. When you say ADE Legacy, I assume you mean
this? Or are you saying that any epub2 file will fail to render that image using that combination of css. Also, in the event of failure, what happens? Does the file not open? Or will the image just not render? Or will it render, but not the way we want it to under condition (b)?