title_body: Paragraph margins: CSS vs br tags
upvoted_answer: Semantically, the most appropriate element to use for section breaks is HR. (Whether it renders properly on reading systems is another matter).  
  
  According to HTML 5, HR is supposed to "indicate  a paragraph-level thematic break, e.g. a scene change in a story, or a transition to another topic within a section of a reference book". https://www.w3.org/TR/html5/grouping-content.html#the-hr-element
  
  HR isn't necessarily a horizontal rule anymore! (this changed from HTML 4)
  
  You can use CSS on the HR to change it from a horizontal line to extra space/longer margins/background image. 
  
  the HTML standard says that br elements must be used only for line breaks that are actually part of the content, as in poems or addresses. br elements must not be used for separating thematic groups in a paragraph.
  
  https://www.w3.org/TR/html5/text-level-semantics.html#the-br-element
  
  In the past I have created classes for p to handle different kinds of section breaks. (I will start using HR instead to do that). 
  
  I would expect that BR rendering is not consistent among reading systems. I don't know if some of the older systems like K4 and below will render HR as a horizontal line or if you can override that. 
  
  Finally about testing on android readers. I have found that some of the minor reading systems apply their own CSS without regard to the ebook's own css. As a result, css rendering is hit-and-miss. Android-wise, I personally test only on Google Play Books, Kindle and Adobe DE. I'm pretty sure all 3 would render CSS for these section breaks (whether they be hr, p or whatever) properly.