title_body: Setting up margins in ePUB and Kindle ebooks and @page rule
upvoted_answer: Quite a few rendering engines are actually taking @page into account. Others like iBooks or Kindle are ignoring it and overriding the margins' values set for body with default values.
  
  I can confirm @page works well in Adobe Digital Editions -- and some but not all apps + devices using Adobe's RMSDK. 
  
  To sum things up, you can use it to set a margin for each page/window/screen while top and bottom margins set for body are just applied to the first (top) and the last (bottom) page of every new (x)HTML file.
  
  As regarding values, don't set them in em in this reflowable world of ours. The more you will increase the font-size, the more those margins will increase as well. 
  
  Px and pt will remain the same, whatever you do. 
  
  % will remain the same and be interpreted as % of screen, which is the most "reflowable-compliant" choice in theory. 3% of a phone screen &lt; 3% of a tablet screen &lt; 3% of a computer screen.