title_body: EPUB 3 CSS stylesheet ignored in some cases?
upvoted_answer: The first thing is: which reading system are you targeting? Edge is just a test environment, right? Or is this how people using Windows devices are reading ebooks these days? I always regard testing in browsers to be at best a preliminary approximation.
  
  I regard Adobe DE and Readium on Chrome as my 2 most reliable testing platform on a Windows machine. Then I try Google Play Books, iBooks  and then test mobis on various kindle platforms. 
  
  I suspect that Edge uses a different rendering engine than the other browsers and might have different default settings for formatting. That might explain discrepancies. But the principles of responsive design dictate that you're not going to have pixel precision. 
  A quick look at your screenshot indicates that what renders in Edge looks pretty close to the other rendering. 
  
  You would need to give the CSS code and what features are different/missing for me to give any better information. It looks like your project is not commercial and maybe that's why you worry about Edge rendering because you think people might open it up in a browser.  But I test a lot of ebooks, and I don't think I've ever tested tested on Edge.