title_body: Is it possible to create a sidebar in a reflowable EPUB3? Is there a way to create a responsive sidebar in EPUB3, for example there would be a sidebar in tablet view but if the EPUB was viewed on a mobile it would become one column?
upvoted_answer: Most epub reading systems do not respond to media queries, so it's going to be hit or miss. You could try putting the sidebar styles first in your CSS, so that in the cascade on systems that don't understand media queries, the sidebar styles will get overwritten by the more general column styles, but no guarantees it will work.
  
  Even on tablets, sidebars may be problematic at high font sizes: just like with tables, if the end of the sidebar does not fit on the current page, it will not continue onto the next page--it will just run off the bottom of the screen.